MXS-3892: Delay routing instead of canceling it

Putting the sessions that aren't doing the update on hold makes the new
mechanism work the same way the old one did with the exception that it
won't put any extra work on the database itself.

server/modules/routing/schemarouter/schemaroutersession.hh

@@ -148,6 +148,7 @@ private:
     int                  inspect_mapping_states(SSRBackend& bref, GWBUF** wbuf);
     enum showdb_response parse_mapping_response(SSRBackend& bref, GWBUF** buffer);
     void                 route_queued_query();
+    bool                 delay_routing(mxb::Worker::Call::action_t action);
     void                 synchronize_shards();
     void                 handle_mapping_reply(SSRBackend& bref, GWBUF** pPacket);
     bool                 handle_statement(GWBUF* querybuf, SSRBackend& bref, uint8_t command, uint32_t type);
@@ -170,5 +171,6 @@ private:
     uint64_t               m_sent_sescmd;   /**< The latest session command being executed */
     uint64_t               m_replied_sescmd;/**< The last session command reply that was sent to the client */
     SERVER*                m_load_target;   /**< Target for LOAD DATA LOCAL INFILE */
+    uint32_t               m_dcid {0};
 };
 }