[FTS] feature enhancement: index only forward-going bucket mutations

Inspired from an off-hand comment from Will...

...it might be interesting to support an FTS index definition flag or mode so that an FTS index only processes forward-going mutations from a bucket. That is, it doesn't look at old documents (never suffers a backfill), but just handles the mutations received after the index was defined.

When used with an FTS index alias, it might be an interesting way to keep indexes small and only focused on the latest, most recent data.

It also might be interesting to use such a thing in conjunction with an older feature of so-called "one-time indexing", where an FTS index is defined to only process up to the seq-numbers at the index definition time.