SPDX standardizes how source code conveys its copyright and licensing
information. See https://spdx.github.io/spdx-spec/1-rationale/ . SPDX
tags are adopted in many large projects, including things like the Linux
kernel.
- Do not use a lambda for std::thread as invoke constructor exists
- Use simpler std::lock_guard wherever possible
- Do not require T to be default constructible
- Move T out of the queue instead of copying
