Avoid using g_rpc_node global in wallet code

Wallet code should use interfaces::Chain and not directly access to node state.

Add a g_rpc_chain replacement global for wallet code to use, and move
g_rpc_node definition to a libbitcoin_server source file so there are link
errors if wallet code tries to access it.

src/rpc/blockchain.cpp

@@ -2289,3 +2289,5 @@ void RegisterBlockchainRPCCommands(CRPCTable &t)
     for (unsigned int vcidx = 0; vcidx < ARRAYLEN(commands); vcidx++)
         t.appendCommand(commands[vcidx].name, &commands[vcidx]);
 }
+
+NodeContext* g_rpc_node = nullptr;