Close binlogrouter connections in thread 0

The internal connections of the binlogrouter should be closed in the same
thread that created them. This should be the "main" thread, i.e. thread 0,
that starts the original binlogrouter service.

server/modules/routing/binlogrouter/blr_slave.c

@@ -3494,6 +3494,7 @@ blr_stop_slave(ROUTER_INSTANCE* router, ROUTER_SLAVE* slave)
         if (router->client->fd != -1 &&
             router->client->state == DCB_STATE_POLLING)
         {
+            // Is this dead code? dcb->fd for internal DCBs is always -1
             dcb_close(router->client);
             router->client = NULL;
         }
@@ -3712,7 +3713,7 @@ blr_start_slave(ROUTER_INSTANCE* router, ROUTER_SLAVE* slave)
     }
 
     /** Start replication from master */
-    blr_start_master(router);
+    blr_start_master_in_main(router);
 
     MXS_NOTICE("%s: START SLAVE executed by %s@%s. Trying connection to master [%s]:%d, "
                "binlog %s, pos %lu, transaction safe pos %lu",