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/httprpc.h

@@ -0,0 +1,37 @@
+// Copyright (c) 2015 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_HTTPRPC_H
+#define BITCOIN_HTTPRPC_H
+
+#include <string>
+#include <map>
+
+class HTTPRequest;
+
+/** Start HTTP RPC subsystem.
+ * Precondition; HTTP and RPC has been started.
+ */
+bool StartHTTPRPC();
+/** Interrupt HTTP RPC subsystem.
+ */
+void InterruptHTTPRPC();
+/** Stop HTTP RPC subsystem.
+ * Precondition; HTTP and RPC has been stopped.
+ */
+void StopHTTPRPC();
+
+/** Start HTTP REST subsystem.
+ * Precondition; HTTP and RPC has been started.
+ */
+bool StartREST();
+/** Interrupt RPC REST subsystem.
+ */
+void InterruptREST();
+/** Stop HTTP REST subsystem.
+ * Precondition; HTTP and RPC has been stopped.
+ */
+void StopREST();
+
+#endif