Support session renaming

[hadronized]

From the beginning of KTS, we’ve always assumed that sessions do not get renamed. That is obviously a wrong assumption and we should move away from that.

A possible solution would be to:

• Upon starting, we provide the session so that KTS knows who we are.
• KTS creates a token for us, and responds with a command to store that token as a set-option global thing. It internally maps the session to that token.
• We use that token instead of the session from now on. KTS knows which session the token maps to so it can reply.
• If the session changes, KTS still receives the same token, but it now needs to talk to the right session. Supporting that probably requires Kakoune to write its current session under a token folder or something to provide the mapping.

The only issue with that is that we cannot really detect dead sessions anymore, besides with KakEnd. If for whatever reason KTS crashes and restarts, it has to be able to restore all the active sessions. We need to think a bit about that problem.

Another alternative would be to allow users to issue a command to reinit after a session rename. That would invalidate the previous session and would restart from scratch, which is a bit a pity, but okay.

[hadronized]

We should get a hook for that ~soon, so we should defer that until we get the feature, it will make support renaming super simple.