The current ThreadPool::Submit is not very efficient when we have a use case where we need to submit multiple tasks immediately. The Submit method must take the lock for each task, and notifies only a single worker thread. This will cause lock contention with the awakened worker thread trying to take the lock and the caller trying to submit the next task.
Introduce SubmitMany, which takes the lock once and submits a vector of tasks, then notifies all worker threads after the lock is released.
This is needed for #31132 to be able to use ThreadPool.
