Split up bitcoinrpc (code movement only)

Split bitcoinrpc up into

- rpcserver: bitcoind RPC server
- rpcclient: bitcoin-cli RPC client
- rpcprotocol: shared common HTTP/JSON-RPC protocol code

One step towards making bitcoin-cli independent from the rest
of the code, and thus a smaller executable that doesn't have to
be linked against leveldb.

This commit only does code movement, there are no functional changes.

src/rpcmining.cpp

@@ -3,7 +3,7 @@
 // Distributed under the MIT/X11 software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
-#include "bitcoinrpc.h"
+#include "rpcserver.h"
 #include "chainparams.h"
 #include "db.h"
 #include "init.h"