[$] Scheduler medley: time-slice extension, sched_ext deadline servers, and LRU batching.

Decades after its creation, the Linux CPU scheduler remains an area
of active development; it is difficult to find a time slice to cover every
interesting scheduler change. In an attempt to catch up, the time has come
to round-robin through a few patches that have been circulating recently.
The work at hand focuses on a new attempt at time-slice extension, the
creation of a deadline server for sched_ext tasks, and keeping tasks on
isolated CPUs from being surprised by LRU batching.