Isolate logic surrounding ls-refs command

[djc]

Rejiggers the code surrouding the ls-refs part of gix-protocol to split pre- and post-transport logic from the core networking logic, with the goal of avoiding code duplication for blocking/async I/O down the line.

[Byron]

Thanks a lot!

This is wonderful and thanks to my additional refactor I think I am also restoring my feeling for this part of the codebase, a part that I have neglected quite a bit, and the happier I am that you are doing this work!

[Byron]

And I forgot to mention: please use the conventional commit prefixes sparingly, particularly with refactor which can be a catch all. Refactors aren't usually relevant in changelogs unless they are breaking. For that reason, I took the liberty of removing all refactor: prefixes from your commits as well.