store: Do not drop the default copy connection when running out of work

When the default worker finished while other workers were still copying,
and there was no more work to do, we inadvertently dropped the default
connection.

Author: lutter

store/postgres/src/copy.rs

@@ -1020,6 +1020,7 @@ impl Connection {
             return None;
         };
         let Some(table) = state.unfinished.pop() else {
+            self.conn = Some(conn);
             return None;
         };