As suggested in #14711, pass height to CChain::FindEarliestAtLeast to
simplify Chain interface by combining findFirstBlockWithTime and
findFirstBlockWithTimeAndHeight into one
Extend findearliestatleast_edge_test in consequence
https://github.com/bitcoin/bitcoin/pull/15670/commits/765c0b364d41e9a251c3f88cbe203645854fd790
Depends on D6256
Backport of Core PR15670