New contract PoA sync fixes (#5991)

* generate proofs on newly-created state

* report only missed steps after first block

* dont report skipped if not signer

* test

* finality tracing and passing valid header to `commit_block`

* avoid proposing multiple times on the same step when validator set changes

* limit benign reports

* Ordering -> AtomicOrdering

* reinstate warning now that spam is reduced

* flush pending transition changes when necessary

* ensure epochs aren't re-zoomed on every block