dcb_write, if write failed, gwbuf wasn't freed.
mysql_backend.c
gw_MySQLWrite_backend, if dcb wasn'r in POLLING state then write was skpiped but gwbuf wasn't freed.
mysql_client.c
gw_MySQLWrite_client, disabled the use of dcb_write because it returns different values than the code it replaced. Also removed erroneous call of gwbuf_consume.
mysql_common.c
mysql_send_custom_error, removed erroneous call of gwbuf_free.