The patch introduces new event subsystem. This subsystem is designed to
store user-defined triggers and has nothing in common with core triggers.

Each trigger has its own name and is represented by func_adapter.
Triggers are stored in events - named wrappers over rlist. Event objects
are opaque, hence rlist field should not be used directly - event
provides event_find_trigger, event_reset_triggers methods and
event_trigger_iterator. Iterator provides stable iteration and all the
non-deleted triggers will surely be traversed.

On way to the goal this patch also fixes include list in func_adapter.h.

Part of #8656

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