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