core: fix crashes after altering trigger list while it is run

This patch fixes a number of issues with trigger_clear() while the
trigger list is being run:
1) clearing the next-to-be-run trigger doesn't prevent it from being run
2) clearing the next-to-be-run trigger causes an infinite loop or a
   crash
3) swapping trigger list head before the last trigger is run causes an
   infinite loop or a crash (see space_swap_triggers() in alter.cc, which
   had worked all this time by miracle: space _space on_replace trigger
   swaps its own head during local recovery, and that had only worked
   because the trigger by luck was the last to run)

This is fixed by adding triggers in a separate run list on trigger_run.
This list may be iterated by `rlist_shift_entry`, which doesn't suffer
from any of the problems mentioned above.

While being bad in a number of ways, old approach supported practically
unlimited number of concurrent trigger_runs for the same trigger list.
The new approach requires the trigger to be in as many run lists as
there are concurrent trigger_runs, which results in quite a big
refactoring.

Add a luatest-based test and a unit test.

Closes #4264

NO_DOC=bugfix