MXS-3054: Prevent pooling of DCBs with inbound data

If a DCB is expecting a response from the server, it must not be placed
into the connection pool. This prevents the case where unexpected
responses would be delivered to DCBs taken out of the connection pool.

server/core/backend.cc

@@ -61,6 +61,7 @@ void Backend::close(close_type type)
             /** Clean operation counter in bref and in SERVER */
             if (is_waiting_result())
             {
+                m_dcb->valid_for_pool = false;
                 clear_state(WAITING_RESULT);
             }
             clear_state(IN_USE);

server/core/dcb.cc

@@ -1260,6 +1260,7 @@ static bool dcb_maybe_add_persistent(DCB* dcb)
         && strlen(dcb->user)
         && server
         && dcb->session
+        && dcb->valid_for_pool
         && session_valid_for_pool(dcb->session)
         && server->persistpoolmax()
         && (server->status & SERVER_RUNNING)