More work on RPC sessions, connections, and managers

This branch doesn't finish the work of making SOCK integrate with RPC (#863 (closed)), but it gets a bit closer, by refactoring session initialization, cleaning up some pending mistakes, and getting our locks a little more like how they should be.

@Diziet i hope this also matches your impression of our conversation the other week? The next step for me after this is to actually provide the traits, types, and downcasting needed to make RPC-socks integration work in socks.rs