adds-changes: generate prev maxrevid if it's missing, small code cleanups

* remove unneeded imports
* write out maxrevid for previous run if it's missing and
  use that for current run boundary
* log class instead of 'if verbose' everywhere
* turn retrieval of current max revid and prev max revid into
  dump steps like everything else

Change-Id: I7fc3a1768b1487e6ee67f9170936a6862033c7bf