Ledger(LedgerHeader const&, Rules, Family&) now starts at immutable_
false: its maps are constructed Synching and filled in afterward by an
acquisition or a replay, so the ledger is only settled by setImmutable()
once both maps are sound. mapHashesFromHeader_ records that this
constructor's hashes are input rather than derived, so setImmutable()
leaves them alone - deriving them from the maps would relabel a map that
fell short of its target instead of refusing it.