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/qt/bitcoin.cpp

@@ -266,13 +266,6 @@ void BitcoinCore::initialize()
     {
         qDebug() << __func__ << ": Running AppInit2 in thread";
         int rv = AppInit2(threadGroup, scheduler);
-        if(rv)
-        {
-            /* Start a dummy RPC thread if no RPC thread is active yet
-             * to handle timeouts.
-             */
-            StartDummyRPCThread();
-        }
         Q_EMIT initializeResult(rv);
     } catch (const std::exception& e) {
         handleRunawayException(&e);
@@ -286,7 +279,7 @@ void BitcoinCore::shutdown()
     try
     {
         qDebug() << __func__ << ": Running Shutdown in thread";
-        threadGroup.interrupt_all();
+        Interrupt(threadGroup);
         threadGroup.join_all();
         Shutdown();
         qDebug() << __func__ << ": Shutdown finished";