tablets: Fix topology_metadata_guard holding on to the old erm

Since abort callbacks are fired synchronously, we must change the
table's erm before we do that so that the callbacks obtain the new
erm.

Otherwise, we will block barriers.

replica/table.cc

@@ -1695,10 +1695,10 @@ table::table(schema_ptr schema, config config, lw_shared_ptr<const storage_optio
 }
 
 void table::update_effective_replication_map(locator::effective_replication_map_ptr erm) {
-    if (_erm) {
-        _erm->invalidate();
+    auto old_erm = std::exchange(_erm, std::move(erm));
+    if (old_erm) {
+        old_erm->invalidate();
     }
-    _erm = std::move(erm);
 }
 
 partition_presence_checker