azync::Connection rework

This MR brings the azync::Connection very close to that of it's synchronous sibling. It now has no generics in the public API, uses separate sockets for reading and writing and uses interior mutability.

Moreover, Connection now is a thin blocking wrapper around azync::Connection. This not only allows us to avoid code-duplication but is inline with my original goal for zbus to have the default API as async. Now I just need to add the highlevel API, which should be much easier now that azync::Connection is almost identical to Connection.