Revert "[Support] Use unique_function rather than std::function for ThreadPool TaskTy."

This reverts commit d9c8b025.

Some MSVC std::packaged_task implementations are not compatible with move-only types.
This caused failures on some of the Windows builders (e.g.
http://lab.llvm.org:8011/builders/sanitizer-windows/builds/69412).

Reverting until I can come up with a workaround.