* Disambiguate an untouched counter from one whose updates accumulate
  to 0 to make `riakc_datatype:prop_unmodified/1` happy
* Related: stop treating a cumulative increment of 0 as a no-op, to
  make both `riakc_map:to_op/1` and riakc_datatype:prop_modified/1`
  happy
* Revert the decision to wrap nested values in datatype records for
  all values; wait until they're needed
* Rework `riakc_map:update/3` and helper functions to be more explicit