Remove a major source of contention in thread_pool

- notify_one when new work is available to avoid thundering herd
  problems
- checking conditions before entering cv_.wait() prevents a case that
  these are not validated

src/cpp/server/thread_pool.cc

@@ -41,6 +41,7 @@ ThreadPool::ThreadPool(int num_threads) {
       for (;;) {
         std::unique_lock<std::mutex> lock(mu_);
         // Wait until work is available or we are shutting down.
+        if (!shutdown_ || callbacks_.empty())
           cv_.wait(lock, [=]() { return shutdown_ || !callbacks_.empty(); });
         // Drain callbacks before considering shutdown to ensure all work
         // gets completed.
@@ -71,7 +72,7 @@ ThreadPool::~ThreadPool() {
 void ThreadPool::ScheduleCallback(const std::function<void()> &callback) {
   std::lock_guard<std::mutex> lock(mu_);
   callbacks_.push(callback);
-  cv_.notify_all();
+  cv_.notify_one();
 }
 
 }  // namespace grpc