YT-21402: Fibers refactoring pt.2: Slightly better encapsulation of fiber introspection part and moved fls back to switch handler

In this pr we refactor fiber state machine so that it is more clear what its purpose is: it protects Fls (and some other fields) across fiber-introspector interaction over the fiber life-cycle and also fix some problems related to the shutdown sequence:
-Fls is back to being created at FiberSwitchHandler (although FiberSwitchHandler is still inappropriately placed in the same UT as fiber scheduler, but that issue is intended to be fixed in later prs).
-FiberId generation is also back to FiberSwitchHandler, with its value being cached inside fiber for introspector use. This shouldn't make things worse, but would make it clearer who *owns* the FiberId and who *views* it.
-TFiberRegistry now uses shutdown callback to clear the last batch of fibers instead of doing so in dtor, which is apparently never called.
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