message: Make iterators const
diff: @@ -42,7 +42,7 @@ AddedRemoved WatchMap::updateWatchInterest(Watch* watch,
  absl::flat_hash_set<Watch*> WatchMap::watchesInterestedIn(const std::string& resource_name) {
  absl::flat_hash_set<Watch*> ret = wildcard_watches_;
  - auto watches_interested = watch_interest_.find(resource_name);
  + const auto watches_interested = watch_interest_.find(resource_name);
  if (watches_interested != watch_interest_.end()) {
  for (const auto& watch : watches_interested->second) {
  ret.insert(watch);
  @@ -73,7 +73,7 @@ void WatchMap::onConfigUpdate(const Protobuf::RepeatedPtrField<ProtobufWkt::Any>
  // We just bundled up the updates into nice per-watch packages. Now, deliver them.
  for (auto& watch : watches_) {
  - auto this_watch_updates = per_watch_updates.find(watch);
  + const auto this_watch_updates = per_watch_updates.find(watch);
  if (this_watch_updates == per_watch_updates.end()) {
  // This update included no resources this watch cares about - so we do an empty
  // onConfigUpdate(), to notify the watch that its resources - if they existed before this -
  @@ -110,7 +110,7 @@ void WatchMap::onConfigUpdate(
  // We just bundled up the updates into nice per-watch packages. Now, deliver them.
  for (const auto& added : per_watch_added) {
  const Watch* cur_watch = added.first;
  - auto removed = per_watch_removed.find(cur_watch);
  + const auto removed = per_watch_removed.find(cur_watch);
  if (removed == per_watch_removed.end()) {
  // additions only, no removals
  cur_watch->callbacks_.onConfigUpdate(added.second, {}, system_version_info);
  