evhttpd implementation

- *Replace usage of boost::asio with [libevent2](http://libevent.org/)*.
boost::asio is not part of C++11, so unlike other boost there is no
forwards-compatibility reason to stick with it. Together with #4738 (convert
json_spirit to UniValue), this rids Bitcoin Core of the worst offenders with
regard to compile-time slowness.

- *Replace spit-and-duct-tape http server with evhttp*. Front-end http handling
is handled by libevent, a work queue (with configurable depth and parallelism)
is used to handle application requests.

- *Wrap HTTP request in C++ class*; this makes the application code mostly
HTTP-server-neutral

- *Refactor RPC to move all http-specific code to a separate file*.
Theoreticaly this can allow building without HTTP server but with another RPC
backend, e.g. Qt's debug console (currently not implemented) or future RPC
mechanisms people may want to use.

- *HTTP dispatch mechanism*; services (e.g., RPC, REST) register which URL
paths they want to handle.

By using a proven, high-performance asynchronous networking library (also used
by Tor) and HTTP server, problems such as #5674, #5655, #344 should be avoided.

What works? bitcoind, bitcoin-cli, bitcoin-qt. Unit tests and RPC/REST tests
pass. The aim for now is everything but SSL support.

Configuration options:

- `-rpcthreads`: repurposed as "number of  work handler threads". Still
defaults to 4.

- `-rpcworkqueue`: maximum depth of work queue. When this is reached, new
requests will return a 500 Internal Error.

- `-rpctimeout`: inactivity time, in seconds, after which to disconnect a
client.

- `-debug=http`: low-level http activity logging

src/rpcprotocol.h

@@ -10,10 +10,6 @@
 #include <map>
 #include <stdint.h>
 #include <string>
-#include <boost/iostreams/concepts.hpp>
-#include <boost/iostreams/stream.hpp>
-#include <boost/asio.hpp>
-#include <boost/asio/ssl.hpp>
 #include <boost/filesystem.hpp>
 
 #include "univalue/univalue.h"
@@ -26,6 +22,7 @@ enum HTTPStatusCode
     HTTP_UNAUTHORIZED          = 401,
     HTTP_FORBIDDEN             = 403,
     HTTP_NOT_FOUND             = 404,
+    HTTP_BAD_METHOD            = 405,
     HTTP_INTERNAL_SERVER_ERROR = 500,
     HTTP_SERVICE_UNAVAILABLE   = 503,
 };
@@ -79,88 +76,6 @@ enum RPCErrorCode
     RPC_WALLET_ALREADY_UNLOCKED     = -17, //! Wallet is already unlocked
 };
 
-/**
- * IOStream device that speaks SSL but can also speak non-SSL
- */
-template <typename Protocol>
-class SSLIOStreamDevice : public boost::iostreams::device<boost::iostreams::bidirectional> {
-public:
-    SSLIOStreamDevice(boost::asio::ssl::stream<typename Protocol::socket> &streamIn, bool fUseSSLIn) : stream(streamIn)
-    {
-        fUseSSL = fUseSSLIn;
-        fNeedHandshake = fUseSSLIn;
-    }
-
-    void handshake(boost::asio::ssl::stream_base::handshake_type role)
-    {
-        if (!fNeedHandshake) return;
-        fNeedHandshake = false;
-        stream.handshake(role);
-    }
-    std::streamsize read(char* s, std::streamsize n)
-    {
-        handshake(boost::asio::ssl::stream_base::server); // HTTPS servers read first
-        if (fUseSSL) return stream.read_some(boost::asio::buffer(s, n));
-        return stream.next_layer().read_some(boost::asio::buffer(s, n));
-    }
-    std::streamsize write(const char* s, std::streamsize n)
-    {
-        handshake(boost::asio::ssl::stream_base::client); // HTTPS clients write first
-        if (fUseSSL) return boost::asio::write(stream, boost::asio::buffer(s, n));
-        return boost::asio::write(stream.next_layer(), boost::asio::buffer(s, n));
-    }
-    bool connect(const std::string& server, const std::string& port)
-    {
-        using namespace boost::asio::ip;
-        tcp::resolver resolver(stream.get_io_service());
-        tcp::resolver::iterator endpoint_iterator;
-#if BOOST_VERSION >= 104300
-        try {
-#endif
-            // The default query (flags address_configured) tries IPv6 if
-            // non-localhost IPv6 configured, and IPv4 if non-localhost IPv4
-            // configured.
-            tcp::resolver::query query(server.c_str(), port.c_str());
-            endpoint_iterator = resolver.resolve(query);
-#if BOOST_VERSION >= 104300
-        } catch (const boost::system::system_error&) {
-            // If we at first don't succeed, try blanket lookup (IPv4+IPv6 independent of configured interfaces)
-            tcp::resolver::query query(server.c_str(), port.c_str(), resolver_query_base::flags());
-            endpoint_iterator = resolver.resolve(query);
-        }
-#endif
-        boost::system::error_code error = boost::asio::error::host_not_found;
-        tcp::resolver::iterator end;
-        while (error && endpoint_iterator != end)
-        {
-            stream.lowest_layer().close();
-            stream.lowest_layer().connect(*endpoint_iterator++, error);
-        }
-        if (error)
-            return false;
-        return true;
-    }
-
-private:
-    bool fNeedHandshake;
-    bool fUseSSL;
-    boost::asio::ssl::stream<typename Protocol::socket>& stream;
-};
-
-std::string HTTPPost(const std::string& strMsg, const std::map<std::string,std::string>& mapRequestHeaders);
-std::string HTTPError(int nStatus, bool keepalive,
-                      bool headerOnly = false);
-std::string HTTPReplyHeader(int nStatus, bool keepalive, size_t contentLength,
-                      const char *contentType = "application/json");
-std::string HTTPReply(int nStatus, const std::string& strMsg, bool keepalive,
-                      bool headerOnly = false,
-                      const char *contentType = "application/json");
-bool ReadHTTPRequestLine(std::basic_istream<char>& stream, int &proto,
-                         std::string& http_method, std::string& http_uri);
-int ReadHTTPStatus(std::basic_istream<char>& stream, int &proto);
-int ReadHTTPHeaders(std::basic_istream<char>& stream, std::map<std::string, std::string>& mapHeadersRet);
-int ReadHTTPMessage(std::basic_istream<char>& stream, std::map<std::string, std::string>& mapHeadersRet,
-                    std::string& strMessageRet, int nProto, size_t max_size);
 std::string JSONRPCRequest(const std::string& strMethod, const UniValue& params, const UniValue& id);
 UniValue JSONRPCReplyObj(const UniValue& result, const UniValue& error, const UniValue& id);
 std::string JSONRPCReply(const UniValue& result, const UniValue& error, const UniValue& id);