Request: expose `flush` to batcher

[dobesv]

If we use the batcher to operate on a bunch of items and we reach the end, it would be useful to "flush" the batcher if there are rows remaining.

Currently if the batcher has a timeout you can wait for that timeout to expire.

However, if there is no timeout there is actually no public API to either retrieve the current batch nor to force the "batch" event to fire.

[wmhartl]

I'll second this request - I'm currently using _flush when catching certain events (SIGINT, SIGTERM) to push a final batch event and that seems "unofficial".