_async
- rejects logg errors, we may want to consider log levels and always log
  errors
MarkeyChildModel:
- handleChanges will update the markers on all of its property changes
  and only render /  hit the gMarkerManager on the last update
- updates to models (deep changes work again)
MarkerParentModel:
- .catches on _async work to reset existingPieces so new work can come
  in and fail.. this way it keeps logging errors instead of just
swallowing them.
uiGmapPromiseSpec - makes sure resolve is working

MarkerManagers - update function to remove then add a marker to the map
issue #904