This is a notification primitive used in GOODS Java API. By means of this
object client's threads can wait notifications about persistence
object instance modification event or transaction abort event.
After signaling, event leaves in signaled state until some thread
invokes waitSignal() method. If waitSignal() method is invoked by thread
before event was signaled, then thread is blocked until some other thread
will call signal() method. If several threads are waiting for event, only
one of them will awakeby signal() method.
Attention: waitSignal() method in general should not be called from
synchronized method because caller object mutex will not be unlocked.