Event vacancy pull doesn't try hard enough to maintain waitlist order

[nbudin]

Imagine the following registration policy:

• Bucket 1: Green, 1 slot, counted
• Bucket 2: Blue, 1 slot, counted

This event currently has the following signups:

• Alice is currently confirmed in the Green bucket, but signed up with no bucket preference (in other words, she randomly ended up in the Green bucket).
• Bob is currently confirmed in the Blue bucket (his bucket preference is irrelevant for this example).
• Carol is currently first on the waitlist, and has requested the Green bucket.
• Dan is currently second on the waitlist with no bucket preference.

Bob withdraws from the event. This leaves a single Blue slot open. What we'd like to have happen is:

1. The app attempts to pull Carol into the game, since she's first on the waitlist. It can't do this, since Carol requested the Green bucket and there are no slots available in it.
2. The app then checks if it can make a Green slot available by moving other signups. It can do so by moving Alice to the Blue bucket, since Alice has no preference. It does that.
3. Now there is a Green slot available, so it pulls Carol into it, leaving Dan first on the waitlist.

What currently happens instead is:

1. The app attempts to pull Carol into the game, since she's first on the waitlist. It can't do this, since Carol requested the Green bucket and there are no slots available in it.
2. Since it can't pull Carol in, it continues down the waitlist and tries to pull Dan into the open Blue slot. Dan has no bucket preference, so it pulls him in, effectively skipping Carol on the waitlist order.