http2: fix several timeout related issues

* correctly reset write timers: currently reset timers on
  both session & stream when write starts and when it ends.
* prevent large writes from timing out: when writing a large
  chunk of data in http2, once the data is handed off to C++,
  the JS session & stream lose all track of the write and will
  timeout if the write doesn't complete within the timeout window
  Fix this issue by tracking whether a write request is ongoing and
  also tracking how many chunks have been sent since the most recent
  write started. (Since each write call resets the timer.)

PR-URL: https://github.com/nodejs/node/pull/16525


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>