Protocol layers

Work in progress; everything is subject to change

This is an ongoing effort to add a mechanism of layered protocols as described in #752 (closed). Currently, there is a new protolayer and session2 API in daemon/session2. The API deviates slightly from the one in the proposal - some parts have been removed because they were unnecessary, some things were added to accommodate for the needs of some protocols, but the gist of how the mechanism works is the same as proposed.

I'm trying to document the API as much as possible, and I'm planning on writing up some kind of an implementation guide for reference for the future implementation of additional protocols (like QUIC).

Individual protocol layer implementations, as well as migration from session to session2, are yet to be done. Protocols are implemented, bugfixing, code cleanups, and documentation are currently the focus of this effort.