http2: send out pending data earlier

If there’s a lot of data waiting on a given stream, send
it out early, if possible. This helps trigger the backpressure
mechanism introduced in 8a4a1931 at a better time.

PR-URL: https://github.com/nodejs/node/pull/29398


Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>