tools/cephfs_mirror: Synchronize taking snapshot

tools/cephfs_mirror: Synchronize taking snapshot

The crawler/entry creation thread needs to wait until
all the data is synced by datasync threads to take
the snapshot. This patch adds the necessary conditions
for the same.

It is important for the conditional flag to be part
of SyncMechanism and not part of PeerReplayer class.
The following bug would be hit if it were part of
PeerReplayer class.

When multiple directories are confiugred for mirroring as below
/d0                /d1              /d2
Crawler1         Crawler2          Crawler3
DoneEntryOps     DoneEntryOps      DoneEntryOps
WaitForSafeSnap  WaitForSafeSnap   WaitForSafeSnap

When all crawler threads are waiting at above, the data sync threads
which is done processing /d1, would notify, waking up all the crawlers
causing spurious/unwanted wake up and half baked snapshots.

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