rt: refactor current-thread scheduler (#4377)

This patch does some refactoring to the current-thread scheduler bringing it closer to the
structure of the multi-threaded scheduler. More specifically, the core scheduler data is stored
in a Core struct and that struct is passed around as a "token" indicating permission to do
work. The Core structure is also stored in the thread-local context.

This refactor is intended to support #4373, making it easier to track counters in more locations
in the current-thread scheduler.