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/init.cpp

@@ -16,6 +16,8 @@
 #include "checkpoints.h"
 #include "compat/sanity.h"
 #include "consensus/validation.h"
+#include "httpserver.h"
+#include "httprpc.h"
 #include "key.h"
 #include "main.h"
 #include "miner.h"
@@ -144,6 +146,15 @@ public:
 static CCoinsViewDB *pcoinsdbview = NULL;
 static CCoinsViewErrorCatcher *pcoinscatcher = NULL;
 
+void Interrupt(boost::thread_group& threadGroup)
+{
+    InterruptHTTPServer();
+    InterruptHTTPRPC();
+    InterruptRPC();
+    InterruptREST();
+    threadGroup.interrupt_all();
+}
+
 void Shutdown()
 {
     LogPrintf("%s: In progress...\n", __func__);
@@ -158,7 +169,11 @@ void Shutdown()
     /// module was initialized.
     RenameThread("bitcoin-shutoff");
     mempool.AddTransactionsUpdated(1);
-    StopRPCThreads();
+
+    StopHTTPRPC();
+    StopREST();
+    StopRPC();
+    StopHTTPServer();
 #ifdef ENABLE_WALLET
     if (pwalletMain)
         pwalletMain->Flush(false);
@@ -424,13 +439,6 @@ std::string HelpMessage(HelpMessageMode mode)
     strUsage += HelpMessageOpt("-rpcport=<port>", strprintf(_("Listen for JSON-RPC connections on <port> (default: %u or testnet: %u)"), 8332, 18332));
     strUsage += HelpMessageOpt("-rpcallowip=<ip>", _("Allow JSON-RPC connections from specified source. Valid for <ip> are a single IP (e.g. 1.2.3.4), a network/netmask (e.g. 1.2.3.4/255.255.255.0) or a network/CIDR (e.g. 1.2.3.4/24). This option can be specified multiple times"));
     strUsage += HelpMessageOpt("-rpcthreads=<n>", strprintf(_("Set the number of threads to service RPC calls (default: %d)"), 4));
-    strUsage += HelpMessageOpt("-rpckeepalive", strprintf(_("RPC support for HTTP persistent connections (default: %d)"), 1));
-
-    strUsage += HelpMessageGroup(_("RPC SSL options: (see the Bitcoin Wiki for SSL setup instructions)"));
-    strUsage += HelpMessageOpt("-rpcssl", _("Use OpenSSL (https) for JSON-RPC connections"));
-    strUsage += HelpMessageOpt("-rpcsslcertificatechainfile=<file.cert>", strprintf(_("Server certificate file (default: %s)"), "server.cert"));
-    strUsage += HelpMessageOpt("-rpcsslprivatekeyfile=<file.pem>", strprintf(_("Server private key (default: %s)"), "server.pem"));
-    strUsage += HelpMessageOpt("-rpcsslciphers=<ciphers>", strprintf(_("Acceptable ciphers (default: %s)"), "TLSv1.2+HIGH:TLSv1+HIGH:!SSLv2:!aNULL:!eNULL:!3DES:@STRENGTH"));
 
     if (mode == HMM_BITCOIN_QT)
     {
@@ -602,6 +610,21 @@ bool InitSanityCheck(void)
     return true;
 }
 
+bool AppInitServers(boost::thread_group& threadGroup)
+{
+    RPCServer::OnStopped(&OnRPCStopped);
+    RPCServer::OnPreCommand(&OnRPCPreCommand);
+    if (!StartHTTPServer(threadGroup))
+        return false;
+    if (!StartRPC())
+        return false;
+    if (!StartHTTPRPC())
+        return false;
+    if (GetBoolArg("-rest", false) && !StartREST())
+        return false;
+    return true;
+}
+
 /** Initialize bitcoin.
  *  @pre Parameters should be parsed and config file should be read.
  */
@@ -990,9 +1013,8 @@ bool AppInit2(boost::thread_group& threadGroup, CScheduler& scheduler)
     if (fServer)
     {
         uiInterface.InitMessage.connect(SetRPCWarmupStatus);
-        RPCServer::OnStopped(&OnRPCStopped);
-        RPCServer::OnPreCommand(&OnRPCPreCommand);
-        StartRPCThreads();
+        if (!AppInitServers(threadGroup))
+            return InitError(_("Unable to start HTTP server. See debug log for details."));
     }
 
     int64_t nStart;