Make Metadata Sync Work on Background Executor

Metadata sync can take several hours for large clusters and database session should be kept open during its execution to finish. This is bad since if the session closes somehow, the operation would not complete. (even worse if citus.metadata_sync_mode = 'nontransactional', which leaves the cluster at an inconsistent state)

As solution we can make use of background executor, which will retry the operation from failed state.