Fix handling of multi-packet queries in readwritesplit

If a prepared statement sends large amounts of data, the target server
where the data is sent will be tracked. The tracked target was not reset
after a multi-packet query was completed and the target itself was used to
check whether the session was processing a multi-packet query.

Changed the check to use the boolean variable instead of the target and
added a reset of the tracked target after a multi-packet query was
completed.

server/modules/routing/readwritesplit/rwsplit_session_cmd.cc

@@ -63,6 +63,7 @@ void process_sescmd_response(RWSplitSession* rses, SRWBackend& backend,
                 if (command == MXS_COM_STMT_PREPARE)
                 {
                     /** Map the returned response to the internal ID */
+                    MXS_INFO("PS ID %u maps to internal ID %lu", resp.id, id);
                     rses->ps_handles[resp.id] = id;
                 }
             }