Inconsistent pack behavior when moving down stairs (and some other leader/follower behavior)

[SanatMishra]

If a "follower" monster comes down the stairs before its leader, it will not be able to find its leader and loses track of it immediately:
goblinpack.zip (wizard mode) as I move from depth 5 to depth 6. If the leader comes down first, the pack is conserved, so this is effectively random.

Additionally, if that follower dies and is not the only follower of its leader, the leader can get its leader flag cleared incorrectly - can get fixed by applying the fixes in #137 to checkForContinuedLeadership. It also might need to call demoteMonsterFromLeadership to make sure monsters that haven't yet arrived on the level break their relationship with the leader.

Might also need to call checkForContinuedLeadership at the end of restoreMonster, when leaders get cleared. If the follower arrives on the level and is the leader's only follower, the leader needs to clear its leader flag when the follower stops following.

But this raises the question of what pack behavior is even supposed to be when it enters a new level - monsters split off from their pack if they fall to a new level away from their leader, but can stick with each other at random when moving down stairs. If followers stay in the pack when moving down stairs, where are they going to wander until their leader arrives?