trigger: introduce tarantool.trigger.on_change triggers

New event named 'tarantool.trigger.on_change' is called when any event
is modified (trigger.set or trigger.del). All the handlers are called with
one argument - name of the modified event. Returned value of each handler
is ignored. Handlers are fired after the event is changed (the event
contains inserted trigger, if any, and does not contain deleted one, if
any). All thrown errors are logged with error level and do not stop
execution of triggers.

Closes #8664

NO_CHANGELOG=later
NO_DOC=later