tools/cephfs_mirror: Add SyncMechanism Queue

tools/cephfs_mirror: Add SyncMechanism Queue

Add a queue of shared_ptr of type SyncMechanism.
Since it's shared_ptr, the queue can hold both
shared_ptr to both RemoteSync and SnapDiffSync objects.
Each SyncMechanism holds the queue for the SyncEntry
items to be synced using the data sync threads.

The SyncMechanism queue needs to be shared_ptr because
all the data sync threads needs to access the object
of SyncMechanism to process the SyncEntry Queue.

This patch sets up the building blocks for the same.

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