Fix the window size increment logic so it does it much more often.
The code increased the window size by MSS * MSS / N, like
in RFC 2581, but it did it only once every N,
so that was like MSS * MSS / N**2.
Now do it all the time, except for isolated packets like keepalives
that aren't using more than one message of the window.
Seems to speed up outbound significantly, without any
noticable increase in stream.sendsBeforeAck.