The previous commit uncovered another bug that was hidden by it:
`maybe_flush_after_async_reply` was sending a message to the wrong
PID. It was sending a message to `self()` meaning to target a buffer
worker, but `self()` in that context is never the buffer worker, it's
the connector's worker.
This change also revealed a race condition where the buffer workers
could stop flushing messages. So we piggy-backed on the atomic update
of the table size count to check if the buffer worker should be poked
to continue flushing. This allows us to get rid of
`maybe_flush_after_async_reply` altogether.
85089a3210