QUIC FUTURE: Add concurrency architecture design document

[hlandau]

This is not for the QUIC Server MVP. It is a design document produced as part of some necessary long term planning for API evolution to support high performance multithreaded I/O in advanced server applications.

As such, it is not a review priority. However, comment and feedback is of course welcomed.

However, the first part of this document introduces relevant concepts which will be used in the QUIC Server MVP in PRs to follow as this is necessary to fix https://github.com/openssl/openssl/issues/24166. You can stop reading at the heading "Architecture" if you don't care about the future stuff and only want to know about 3.4-relevant things.

The potential architecture for a Concurrency Management Layer (which enables both WCM and non-WCM concurrency models to be implemented without having to maintain two radically different code paths) discussed here is a first draft under contemplation and might change substantially further down the line. It is intended as a preview of some internal architectural designs which are being considered.