Restore thread-wise task schedule reporting in scheduler state dumps

This MR is a refurbished variant of !567 (closed).

@clemenci: I had to close !567 (closed), since it became almost "irrebaseable" after !541 (merged) - its base - was itself rebased several times.

Thread-wise task schedule in-failure/in-stall reporting, which was temporarily disabled in !288 (merged), is now resurrected and reimplemented. The new (shorter) implementation is based on the TimelineSvc and thus is activated only if the latter is enabled (unlike in the previous implementation, where the corresponding tracing calls were creating unconditional overhead). The old implementation is removed.

I also took this occasion to further consolidate the scheduler state dumps, making them a bit less chaotic.