Implement upr expiration command

We don't support doing anything different than a deletion within
the enigne yet so whenever we see and expiration we should just
treat it as a deletion.

Change-Id: I709ba05e7cdfa1800a62d0a54143c991e8787f96
Reviewed-on: http://review.couchbase.org/38338
Reviewed-by: abhinav dangeti <abhinav@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>