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/init.cpp

@@ -10,7 +10,7 @@
 #include "init.h"
 
 #include "addrman.h"
-#include "bitcoinrpc.h"
+#include "rpcserver.h"
 #include "checkpoints.h"
 #include "miner.h"
 #include "net.h"