Support multi-threaded JSON-RPC

Change internal HTTP JSON-RPC server from single-threaded to
thread-per-connection model.  The IP filter list is applied prior to starting
the thread, which then processes the RPC.

A mutex covers the entire RPC operation, because not all RPC operations are
thread-safe.

[minor modifications by jgarzik, to make change upstream-ready]

src/bitcoinrpc.h

@@ -9,6 +9,7 @@
 #include <string>
 #include <map>
 
+#define BOOST_SPIRIT_THREADSAFE
 #include "json/json_spirit_reader_template.h"
 #include "json/json_spirit_writer_template.h"
 #include "json/json_spirit_utils.h"