[runtime] Don't return from Monitor.Wait without holding the lock

[runtime] Don't return from Monitor.Wait without holding the lock

Even if we are interrupted we must throw the exception only after we have regained the lock. This prevents throwing an SynchronizationLockException in the finally clause of common scenarios like :

lock (obj) {
	Monitor.Wait (obj);
}

The downside of this is that, if another thread doesn't give up the lock, we could fail to abort the thread. But this is what MS does.