Revert "core: implement rise-fall counting for overlapping pulses to be handled correctly."

This reverts commit d1aac1bdc8fb39afff4e1a30853aba9b6b51b943.

Conflicts:
	core.h
core: unify lock_guard_t definitions.
common, sources/frequency: unify clock typedefs.
core: set default handler for intercepted signal to allow force-terminate via second raise of the same signal.
core: rework threading, part 1: do not fire off a thread for each pulse.

Instead, create a worker thread for each channel that is active and add pulse
requests to the per-worker queue.
core: consider exiting from a worker thread only after emptying the queue.

This ensures that every worker thread terminates with channel brought down, and
so the core itself leaves sinks in "clean" state on exit.
core: (cosmetics) make set_destroying() a no-op on consequent calls.