delete limited expired docs in every TTL period

TTL monitor delete all the expired docs every 60 seconds.
Sometimes a few expired docs exist in one TTL monitor period, and lots of expired docs in the next TTL monitor period. Then a big performance degration happens as too many delete requests appears in a short time.

If 'ttlDeleteBatch' is supported, we can delete a limited number of docs in one period which is under control, then no performance degration will happen.