[scheduler] Better Semantics and Naming for `SchedulerHandle` Functions

[ppenna]

Description

Currently, SchedulerHandle is used to refer to both short-living and long-living operations.

The problem here is that each of these take different life-cycle paths and end up having different semantics on when their underlying reference counting is updated on WakerPageRefs.

This design does not expose a clear interface and function names do not help either.