*debug-tasks-p* does not respect current *debugger-hook* changes

[tdrhq]

Notice that in: https://github.com/sharplispers/lparallel/blob/master/src/kernel/handling.lisp#L112

It holds on to the *debugger-hook*... but this is the debugger hook when the kernel was started.

If you connect to this server via SLY/Slime, the *debugger-hook* changes, but lparallel will still be using the old debugger-hook.

This means that there's no way to debug with SLY/Slime.

Thoughts on fixing:

• We could use (symbol-value '*debugger-hook*) to get the global value instead
• I'm not convinced this should be modifying *debugger-hook* at all, and should instead be using handle-case/handler-bind directly

Happy to help with the change, but I'd love to see if people have specific thoughts on this and if there are edge cases I'm not thinking about