Move resumability decisions from the client to the server

[SimonWoolf]

Implements https://ably.atlassian.net/wiki/spaces/CHN/pages/5078777859/PSRFC-32+move+resumability+decisions+to+the+server+not+the+sdk

Motivation, in short:

• we can get rid of ATTACH_RESUME, which is weird and confusing
• it gives us the freedom to experiment with providing longer resume windows, streaming resumes from new-model channel history, with purely serverside logic
• it allows us to start decoupling “how long we can resume subscriptions for” from “how long we can keep a message queued for publishing”, there's no fundamental reason those two should be tied together

(also tweaked docstrings for connection states, which were just the full wording from the docs copied and pasted, and were way too verbose for docstrings)