Fix to bug #541, http://bugs.skysql.com/show_bug.cgi?id=541

Long ~0.5MB queries blocked MaxScale.
mysql_client.c:gw_read_client_event: Fixed packet reading logic. Reading didn't work when packet exceeded read buffer size.
mysql_common.c:gw_MySQL_get_next_packet: number of bytes to be copied to continuous buffer was calculated wrong, thus resulting in broken packet.
readwritesplit.c:disabled creation of canonical query in debug build because it slows down the processing too much with long queries

server/modules/routing/readwritesplit/readwritesplit.c

@@ -1949,7 +1949,7 @@ static int routeQuery(
 	}
 	rses_end_locked_router_action(router_cli_ses);
 retblock:
-#if defined(SS_DEBUG)
+#if defined(SS_DEBUG2)
         {
                 char* canonical_query_str;