sched: Refactor and fix interactions between channel and scheduler
This is a top level ticket for all the things we need to address in the channel subsystem and scheduler.
Channels interact with the scheduler using a few entry points and assumptions provided by the circuit and channel subsystem. However, it turns out that the channel abstraction is often violated from the connection subsystem with channel_t and channeltls_t objects.
We also have too many cells queues that goes from connection inbuf to circuit queue to channel queue to connection outbuf. But #23709 (moved) showed us that the channel queues aren't quite working as intended.
Furthermore, we put scheduler state data in the channel_t object that are actually only relevant to the Vanilla scheduler but not to the other types (KIST and KISTLite for now).
Along with pastly, we've discussed changes to simplify the scheduler interface, to make scheduler data stored in a channel tied to a scheduler type and make some changes to improve the channel subsystem.
The child tickets address all of the above (and maybe more) in order to have a leaner channel subsystem that actually provides properties and correct assumptions to the scheduler.