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/bitcoind.cpp

@@ -10,11 +10,16 @@
 #include "noui.h"
 #include "scheduler.h"
 #include "util.h"
+#include "httpserver.h"
+#include "httprpc.h"
+#include "rpcserver.h"
 
 #include <boost/algorithm/string/predicate.hpp>
 #include <boost/filesystem.hpp>
 #include <boost/thread.hpp>
 
+#include <stdio.h>
+
 /* Introduction text for doxygen: */
 
 /*! \mainpage Developer documentation
@@ -44,7 +49,7 @@ void WaitForShutdown(boost::thread_group* threadGroup)
     }
     if (threadGroup)
     {
-        threadGroup->interrupt_all();
+        Interrupt(*threadGroup);
         threadGroup->join_all();
     }
 }
@@ -154,7 +159,7 @@ bool AppInit(int argc, char* argv[])
 
     if (!fRet)
     {
-        threadGroup.interrupt_all();
+        Interrupt(threadGroup);
         // threadGroup.join_all(); was left out intentionally here, because we didn't re-test all of
         // the startup-failure cases to make sure they don't result in a hang due to some
         // thread-blocking-waiting-for-another-thread-during-startup case