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.
Technically, the block validator caught this panic. But it's pointless
because we have a _real_ mechanism to return the validation reason,
which we should have been using.
In general, panicing like this is a very bad idea because it's
non-obvious and, in this case, completely undocumented.
