Consider more actively flushing email batch on "needs work" or "approved"

Original summary: "Batched mail too slow, unbatched mail too noisy."

We recently upgraded from 2.12 to 4.5 and our users are complaining because the batch feature, which they were looking forward to, significantly slows down mail delivery (they're stuck with either getting mail slowly or getting feedback which is later removed and having it dribble in.

As mentioned in BSERV-2839, what they'd like is for the mail to be delivered immediately, but only after the reviewer has finished reviewing (preferably, a single mail that includes all the feedback, rather than multiple mails).

The goals are:

• a reviewers comments should not arrive before they're finished reviewing all the code
• minimize email
• mail should arrive as fast as possible after the reviewer has finished reviewing the code
• comments which are later found to be incorrect and are removed by the reviewer should not generate mail