Set event state to something sensible after planned maintenance ends

[stveit]

Right now all events affected by a planned maintenance will be forced to open after the pm ends. Comment from zino1 code:

    # when a PM period ends, take the affected interfaces out of
    # "ignored" state and back into (?) "open" or "closed"(?)
    # state, possibly depending on whether the interface is back
    # up or remains down(?).

    # Similar for reachability and alarm PM events

    # For now just set state unconditionally back to "open", and
    # leave the closing of the events to be done manually

Something should be changed in how this is done, but not sure exactly how the behaviour should be