http: overridable keep-alive behavior of `Agent`

Introduce two overridable `Agent` methods:

* `keepSocketAlive(socket)`
* `reuseSocket(socket, req)`

These methods can be overridden by particular `Agent` class child to
make keep-alive behavior customizable.

Motivation: destroy persisted sockets after some configurable timeout.
It is very non-trivial to do it with available primitives. Such program
will most likely need to poke with undocumented events and methods of
`Agent`. With introduced API such behavior is easy to implement.

PR-URL: https://github.com/nodejs/node/pull/13005


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>