MB-8012 Compactor can do timestamp-based purge

Passing --purge-before <timestamp> will drop any deletion records with
an expiration time less than the timestamp.

Change-Id: I2d4d903f7c83d53269b4394d0e80385d75e1c199
Reviewed-on: http://review.couchbase.org/27426
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Reviewed-by: Fulu Li <fulu@couchbase.com>
Tested-by: Aaron Miller <apage43@ninjawhale.com>
Merge branch '2.1.1'

Conflicts:
	src/db_compact.cc

Change-Id: I8cf3820220adbab5998b0df5e63d0a60356551e2