Motivation:

To dedupe the thread pool from NIOFileSystem and NIOPosix, the one from
NIOPosix needs to gain support for cancellation for its async
`runIfActive` function.

Modification:

- Generate a work ID when submitting async work to the pool, add that ID
  to a set in the cancellation handler
- Check the existence of the ID in the cancel set when dequeuing the
  work

Result:

Queued NIOThreadPool tasks can be cancelled before they are run.