This reverts commit cf27a6bb76.
The use of "shared_lock" in TSLQueue is unsafe because of two things:
- The TSLQueue iterator takes a "read" lock.
- The TSLQueue iterator can erase the current element.
This project supports C++11, and std::shared_lock was made available in
C++17, thus a "shared_spin_lock" was created with similar functionality.
This "shared_spin_lock" is used in TSLQueue.
