raft: invoke update triggers within state machine

Raft used to call on_update trigger from the worker fiber. It was
done because it could yield. But it is not the case anymore. The
only yielding operation was box_clear_synchro_queue(), which is
not called from the trigger now.

That makes possible to call the trigger from within of the state
machine. And this removes the yield between the raft state change
and the trigger invocation.

What, in turn, allows to move all box-related urgent updates to
the trigger. Such as box_update_ro_summary().

Part of #5303