[RPC, Wallet] Move RPC dispatch table registration to wallet/ code

Allow extending the rpc dispatch table by appending commands when server is not running.

src/wallet/rpcwallet.h

@@ -0,0 +1,10 @@
+// Copyright (c) 2016 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_WALLET_RPCWALLET_H
+#define BITCOIN_WALLET_RPCWALLET_H
+
+void walletRegisterRPCCommands();
+
+#endif //BITCOIN_WALLET_RPCWALLET_H