core: get rid of trigger_fiber_run function

Function trigger_fiber_run is used only for on_shutdown triggers and
uses internal structure run_list. This structure is another list but all
the triggers are popped from run_list instead of iteration because this
approach is safe when triggers are deleted from the list that is being run.
Also, new triggers are not inserted to run_list.

Since we are running only on_shutdown triggers, which won't be used after
they are fired, we can move all the triggers to an internal trigger
list (so that no new triggers will appear) and pop them instead of
iteration. So let's remove function trigger_fiber_run and run on_shutdown
core triggers in a new special function. Later, this new function will run
triggers from on_shutdown event as well.

Part of #8657

NO_TEST=no behavior changes
NO_CHANGELOG=later
NO_DOC=later