REGRESSION: inconsistent constructMap order in meshToMesh::calcProcMap

- commit fb69a54b accidentally changed the constructMap compact
  order from linear ordering to local elements first order. Seems to
  interact poorly with other bookkeeping so doing a partial revert,
  but still replacing the old allGatherList with exchangeSizes.

Note:
   the processorLOD method does actually use a constructMap with local
   elements first ordering, so some inconsistency may still exist
   there