Send stream-level sendme when outbuf is low: don't wait for it to empty!
Right now we consider sending an stream-level SENDME under two conditions:
- We get a data cell on that stream.
- We have completely flushed the outbuf for that stream.
But suppose that the sender has exhausted its quota of cells. We actually are willing to send a SENDME cell before we've flushed everything down to 10*RELAY_PAYLOAD_SIZE (and we should, for throughput). So we should add another check that happens when we flush data, to see if we have drained enough to send a stream-level relay cell.