Unwrap Mutex from broken_link_callback

[liamwhite]

While working on #481 I realized that the same principle of making a dyn trait instead of a FnMut could be applied to the broken link callback. This PR implements an API break for that, which does the following:

• removes the Mutex wrapper, allowing the link callback to be lock-free if the underlying function is also lock-free
• makes it easier write the callback if it is a plain Fn (you can still use interior mutability within like Arc<Mutex> if you need it)
• replaces a more complicated explicit Debug impl with a single simpler explicit Debug impl
• simplifies a little bit of the lifetime soup from the Options struct

[kivikakk]

Thank you! I plan to review this and your other PR this week; been flat out!

[kivikakk]

Thank you, this is awesome!