Add holding replication snapshots - #29

[kapitainsky]

Without holding snapshots when destination dataset is missing (example can be external disk) script can accidentally delete last common snapshot - as can not compare source and destination.

The solution is to hold common snapshots using "zfs hold keep":

https://www.thegeekdiary.com/understanding-holding-a-zfs-snapshot-feature/

I have added required logic and did testing.

[kapitainsky]

I tested with very aggressive snapshot retention policy and it works beautifully. When I remove destination dataset and keep creating new snapshots, pruning source dataset snapshots always preserves snapshot needed for future replication - it would be pruned otherwise breaking replication:

Deleting source snapshot: tank/iocage@zfsbud_20230425151851 <<--- on hold
Deleting source snapshot: tank/iocage@zfsbud_20230425151949
Deleting source snapshot: tank/iocage@zfsbud_20230425151951
Deleting source snapshot: tank/iocage@zfsbud_20230425151953
Deleting source snapshot: tank/iocage@zfsbud_20230425151955

When replication happens at some stage the hold is released (and new one is set on latest snapshots) and subsequent prunning gets rid of old snapshot.