MB-8012 Accept purge-before option for compactor

Adds a {purge_before, Timestamp} option, where Timestamp is a unix
timestamp in seconds.

Change-Id: I5ee537dc04ddcd17dfd54d1f1095a84943e1c76a
Reviewed-on: http://review.couchbase.org/27428
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Aaron Miller <apage43@ninjawhale.com>