MB-8780: added compaction option to avoid purging too new seqnos

Change-Id: I0dcdba09c276ca233add69446947e8f8906905fc
Reviewed-on: http://review.couchbase.org/28034
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aaron Miller <apage43@ninjawhale.com>
Tested-by: Aaron Miller <apage43@ninjawhale.com>