Base implementation of /datum/persistent_client

[Absolucy]

About The Pull Request

Ports the following PRs from tg:

• Base implementation of /datum/persistent_client tgstation/tgstation#89449
• makes get_mob_by_ckey not horrifically inefficient tgstation/tgstation#90401
  • already partially ported, this just adds the additional optimization using persistent clients

Converts /datum/player_details into /datum/persistent_client.
Persistent Clients persist across connections. The only time a mob's persistent client will change is if the ckey it's bound to logs into a different mob, or the mob is deleted (duh).

Also adds PossessByPlayer() so that transfering mob control is cleaner and makes more immediate sense if you don't know byond-fu.

Why It's Good For The Game

Clients are an abstract representation of a connection that can be dropped at almost any moment so putting things that should be stable to access at any time onto an undying object is ideal. This allows for future expansions like abstracting away client.screen and managing everything cleanly.

Changelog

No player-facing changes