Allow temporarily skipping the creation of remote writer for SynchronousReplicationWALProvider

If the remote cluster is broken and we want to downgrade the local cluster to not write WAL to remote cluster, we need to reopen the region to switch back to use the normal WAL.

But we need to flush before reopening a region, and the flush need to write a flush marker to WAL. Since the WAL has already been broken, this can not be done...