replication: update last_sync before removing old replication snapshots

replication: update last_sync before removing old replication snapshots

If pvesr was terminated after finishing with the new sync and after
removing old replication snapshots, but before it could write the new
state, the next replication would fail. It would wrongly interpret the
actual last replication snapshot as stale, remove it, and (if no other
snapshots are present) attempt a full sync, which would fail.

Reported in the community forum [0], this was brought to light by the
new pvescheduler before it learned graceful reload.

It's not possible to simply preserve a last remaining snapshot in
prepare(), because prepare() is also used for valid removals. Instead,
update last_sync early enough. Stale snapshots will still be removed
on the next run if there are any.

[0]: https://forum.proxmox.com/threads/100154

Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>