- provides mechanism to keep WebSockets from spamming the $digest loop
- flips invokeApply passed to the internal $timeout calls,
  stopping a $digest loop & avoiding a render.
- DataService._watchOpOnMessage also honors the skipDigest flag,
  which is plumbed down through .watch() -> .startListOp -> ._listOpComplete -> ._startWatchOp -> ._watchOpOnMessage

example usage:

DataService.watch(resource, context, (data) => {
  // optionally call $applyAsync here in controlling code
  // to manually run a $digest
  $scope.$applyAsync(() => {
   // do stuff with data here
  });
}, { skipDigest: true });

// or Debounce
DataService.watch(resource, context, _.debounce(() => {
 // $apply here, inside debounce
}, 400), { skipDigest: true });