MXS-1178: Fix master_accept_reads

The order of the servers in the service definition could break the
master_accept_reads functionality.

When the first server defined in the service is a slave, it will always be
picked as the first candidate for reads. The master would only be
considered as a candidate for reads if no previous candidate was
available. For this reason, the master_accept_reads only worked when the
first server in the list was the master.

server/modules/routing/readwritesplit/readwritesplit.c

@@ -1236,7 +1236,8 @@ static bool get_dcb(DCB **p_dcb, ROUTER_CLIENT_SES *rses, backend_type_t btype,
              * backend and update assign it to new candidate if
              * necessary.
              */
-            else if (SERVER_IS_SLAVE(&server))
+            else if (SERVER_IS_SLAVE(&server) ||
+                     (rses->rses_config.rw_master_reads && SERVER_IS_MASTER(&server)))
             {
                 if (max_rlag == MAX_RLAG_UNDEFINED ||
                     (b->backend_server->rlag != MAX_RLAG_NOT_AVAILABLE &&