http2: track memory allocated by nghttp2

Provide a custom memory allocator for nghttp2, and track
memory allocated by the library with it.

This makes the used-memory-per-session estimate more
accurate, and allows us to track memory leaks either
in nghttp2 itself or, more likely, through faulty
usage on our end.

It also allows us to make the per-session memory limit
more accurate in the future; currently, we are not
handling this in an ideal way, and instead let nghttp2
allocate what it wants, even if that goes over our limit.

PR-URL: https://github.com/nodejs/node/pull/21374
Refs: https://github.com/nodejs/node/pull/21373
Refs: https://github.com/nodejs/node/pull/21336


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>