http2: use session not socket timeout, tests

This fixes https://github.com/nodejs/node/issues/15158. Two changes included:

• there's no longer a default server socket timeout, it's now replaced by a default session timeout
• _unrefActive was being called on the handler rather than the actual session

Marked as WIP as I think we need to investigate whether socket timeout is necessary anywhere and also because this needs a lot more tests, ideally.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

Affected core subsystem(s)

http2, test