MB-8012 Store time of deletion in expiry field

Using this timestamp the compactor can purge delete records once they
reach a certain age.

Change-Id: Ib541de9e1a5c14674eaabbea6c03629f9f713870
Reviewed-on: http://review.couchbase.org/27427
Reviewed-by: Chiyoung Seo <chiyoung.seo@gmail.com>
Tested-by: Chiyoung Seo <chiyoung.seo@gmail.com>