src: make CleanupHandles() tear down handles/reqs

Previously, handles would not be closed when the current `Environment`
stopped, which is acceptable in a single-`Environment`-per-process
situation, but would otherwise create memory and file descriptor
leaks.

Also, introduce a generic way to close handles via the
`Environment::CloseHandle()` function, which automatically keeps
track of whether a close callback has been called yet or not.

Many thanks for Stephen Belanger for reviewing the original version of
this commit in the Ayo.js project.

Refs: https://github.com/ayojs/ayo/pull/85
PR-URL: https://github.com/nodejs/node/pull/19377
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>

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