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/net.h

@@ -92,11 +92,12 @@ enum threadId
     THREAD_OPENCONNECTIONS,
     THREAD_MESSAGEHANDLER,
     THREAD_MINER,
-    THREAD_RPCSERVER,
+    THREAD_RPCLISTENER,
     THREAD_UPNP,
     THREAD_DNSSEED,
     THREAD_ADDEDCONNECTIONS,
     THREAD_DUMPADDRESS,
+    THREAD_RPCHANDLER,
 
     THREAD_MAX
 };