storage_service: Notify topology state machine after applying schema change

Table construction may allocate tablets which may need rebalancing.
Notify topology change coordinator to invoke the load balancer.

service/storage_service.cc

@@ -5083,6 +5083,7 @@ void storage_service::on_update_tablet_metadata() {
     }
     // FIXME: Avoid reading whole tablet metadata on partial changes.
     load_tablet_metadata().get();
+    _topology_state_machine.event.signal(); // wake up load balancer.
 }
 
 future<> storage_service::load_tablet_metadata() {