MXS-1047: Fix batch insert execution

Doing batch inserts though readwritesplit would stall due to the fact that
pending session commands were stored instead of executed immediately.

Session command responses that weren't complete also discarded the partial
event instead of storing it for later use.

server/modules/protocol/MySQL/MySQLBackend/mysql_backend.c

@@ -848,6 +848,8 @@ gw_read_and_write(DCB *dcb)
              */
             if (!sescmd_response_complete(dcb))
             {
+                stmt = gwbuf_append(stmt, read_buffer);
+                dcb->dcb_readqueue = gwbuf_append(stmt, dcb->dcb_readqueue);
                 return 0;
             }