message: Introduce a reader lock into ResourceMgr::DoLookup() to reduce contention.
diff: @@ -157,7 +157,7 @@ Status ResourceMgr::DoCreate(const string& container, TypeIndex type,
  Status ResourceMgr::DoLookup(const string& container, TypeIndex type,
  const string& name,
  ResourceBase** resource) const {
  - mutex_lock l(mu_);
  + tf_shared_lock l(mu_);
  const Container* b = gtl::FindPtrOrNull(containers_, container);
  if (b == nullptr) {
  return errors::NotFound("Container ", container, " does not exist.");
  