Expose underlying Listener thread handle

Using the listener construct, it would be nice to get hold of the worker thread and pin it on startup (etc) without resorting to reinterpret_cast hacks.

-Or is it that outside the intended use case for listener maybe ?

Best,

Start by locating the listener construct and how its worker thread is owned or exposed. Check the issue discussion for the intended API boundary and any objections, then identify the relevant listener tests or documentation; done means the supported way to obtain the thread handle is defined and verified without reinterpret_cast hacks.

Written by the indexing model from the issue text.