Make `get_outputs_to_watch` return a `Vec` instead of a `HashMap`

`get_outputs_to_watch` returned a reference to an existing
`HashMap` avoiding extra clones, but there isn't a huge reason to
do so now that we have to clone to copy it out of the
`ChannelMonitor` mutex. Instead, return a `Vec` since it may be
less memory and it allows us to have a bindings C mapping for the
function.

Co-authored-by: Jeffrey Czyz <jkczyz@gmail.com>
Co-authored-by: Matt Corallo <git@bluematt.me>