Removed unsafe (for backup) use of zfs send -R.

The call to optimize_recursivize() in sync.py is commented out for now,
to be deleted later if this change proves satisfactory.

To purge snapshots on the destination replica, use zreplicate --clear-obsolete,
or drive this from ZFS properties with zbackup running locally on the
destination server.
Restored *tier*-snapshot-limit property.

Now that we're not using zfs send -R, properties do not get
replicated.  So there needs to be a way to configure snapshot reaping
on the destination replica.  This is it.