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

Change-Id: Ibb3fd6702573220f7e1de4d31f0b5286f202e8ae
Reviewed-on: http://review.couchbase.org/28035
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aaron Miller <apage43@ninjawhale.com>