Due to a (now consensus critical) bug in randomness selection, "beacon"
randomness depends on whether the epoch in which the beacon occurred was
a null block. Unfortunately, this means:
1. If a challenge lands on a non-null round but that non-null round gets
re-orged to a null round, the randomness will change to the beacon in
the first preceding non-null block.
2. If a challenge lands on a null round but that null round gets
re-orged to a non-null round, the randomness will change to the beacon
in the new non-null round.
The correct solution is to change the way the beacon is selected:
1. Wait until a non-null round.
2. The block in this round will contain all beacons from the null
rounds, select the beacon from the target epoch.
Unfortunately, this requires a mandatory network upgrade.
Instead, this patch just waits a bit before beginning to compute window
post after passing the challenge round to wait-out any reorgs.
