http2: fix stream reading resumption

_read should always resume the underlying code that is attempting
to push data to a readable stream. Adjust http2 core code to
resume its reading appropriately.

Some other general cleanup around reading, resuming & draining.

PR-URL: https://github.com/nodejs/node/pull/16580
Fixes: https://github.com/nodejs/node/issues/16578


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