src: use `unordered_set` instead of custom rb tree

Use a standard hash-based container instead of the custom included
red/black tree implementation. There is likely no noticeable
performance difference, and if there is one, it is very likely
to be an improvement.

PR-URL: https://github.com/nodejs/node/pull/14826


Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>