[FTS] returns expired items in search results

After conversation with Matt Ingenthron it was pointed out that due to the way expiry process happens in Couchbase Server k/v nodes, expired items may not show up as deletes on the DCP feed for some time.  During this time, they will still be returned as search results, even though on the k/v side they are now considered expired.

Short term, could offer an option to index the expiration, users could then choose to add a clause with restrictions on this field.

Longer term, discussion around what the default behavior should be (indexing expiration field is relatively expensive in FTS), but not returning expired items seems like the sane default...