Avoid unnecessarily cloning unsigned Transaction when broadcasting

Avoid unnecessarily cloning unsigned Transaction when broadcasting

Our `Trusted*` wrappers in `chan_utils` expose additional inner
fields by reference. However, because they were not explicitly
marked as returning a reference with the wrapped struct's
lifetimes, rustc was considering them to return a reference with
the wrapper struct's lifetime.

This is unnecessarily restrictive, and resulted in the addition of
a clone in 9850c5814abdf69883b51a4afe73912ff5436fd9 which we remove
here.