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/test/rpc_tests.cpp

@@ -1,4 +1,5 @@
-#include "bitcoinrpc.h"
+#include "rpcserver.h"
+#include "rpcclient.h"
 
 #include "base58.h"