http, async_hooks: Consider async_hooks implications of the API that HTTP Agent uses

[refack]

• Version: master
• Platform: *
• Subsystem: http, async_hooks

The HTTP Agent is designed to be inherited and it's methods overridden. Currently there is non encapsulated async_hooks logic that can break if not re-implemented in overridden methods. We should better encapsulate this logic, and document for custom Agent implementors.

Ref: #13045
Ref: #13325

/cc @nodejs/async_hooks

[AndreasMadsen]

Ref: #13539

[Trott]

@refack Should this remain open? (I believe the answer is "yes" but I'm checking because it's been inactive for a long time....)

[apapirovski]

Linked issues are closed. PRs landed. As far as I can tell, this is mostly addressed. If something isn't, please feel free to reopen but do provide more info on what exactly is missing or needs to be done.