This will reject blocks in pubsub validation if they're either:
1. Too far into the future (5 blocks beyond the expected head).
2. Too far into the past (before finality with respect to our current
head).
Specifically:
1. We were previously rejecting future blocks in the sync logic, but not
in pubsub itself.
2. We never used to check if a block was too _old_.
Motivation: Blocks that are too new/too old can cause us to perform
quite a bit of unnecessary work.
Steven Allen
784214ae05