tools/cephfs_mirror: Fix data sync threads completion logic

tools/cephfs_mirror: Fix data sync threads completion logic

We need to exactly know when all data threads completes
the processing of a syncm. If a few threads finishes the
job, they all need to wait for the in processing threads
of that syncm to complete. Otherwise the finished threads
would be busy loop until in processing threads finishes.

And only after all threads finishes processing, the crawler
thread can be notified to take the snapshot.

Fixes: https://tracker.ceph.com/issues/73452
Signed-off-by: Kotresh HR <khiremat@redhat.com>