When hard fork happens, check that transition point matches lower bound in EraParams

[edsko]

If it doesn't match, the node is configured incorrectly, and will almost certainly not operate correctly. We can detect it the moment that the transition becomes known; this then gives us a stability window (12 hours for Byron) to detect the problem, so we should trace this with "critical" severity and then hope that devops see it and fix the problem before the hard fork happens.

If the problem persists at the hard fork, I'm not entirely sure what to do. Shutting down would be the safest -- having this value be wrong could have unforeseen consequences in lots of places...?

Marking this as medium priority because it's not critical for a release, but it will avoid some confusion in the tests so assigned it to the current milestone.