node: Add reference to mempool in NodeContext

Currently it is an alias to the global ::mempool and should be used as
follows.

* Node code (validation and transaction relay) can use either ::mempool
  or node.mempool, whichever seems a better fit.
* RPC code should use the added convenience getter EnsureMempool, which
  makes sure the mempool exists before use. This prepares the RPC code
  to a future where the mempool might be disabled at runtime or compile
  time.
* Test code should use m_node.mempool directly, as the mempool is always
  initialized for tests.

src/rpc/protocol.h

@@ -63,6 +63,9 @@ enum RPCErrorCode
     RPC_CLIENT_INVALID_IP_OR_SUBNET = -30, //!< Invalid IP/Subnet
     RPC_CLIENT_P2P_DISABLED         = -31, //!< No valid connection manager instance found
 
+    //! Chain errors
+    RPC_CLIENT_MEMPOOL_DISABLED     = -33, //!< No mempool instance found
+
     //! Wallet errors
     RPC_WALLET_ERROR                = -4,  //!< Unspecified problem with wallet (key not found etc.)
     RPC_WALLET_INSUFFICIENT_FUNDS   = -6,  //!< Not enough funds in wallet or account