The callback-based method of listing remote references dates back to the
beginning of the network code's lifetime, when we didn't know any
better.
We need to keep the list around for update_tips() after disconnect() so
let's make use of this to simply give the user a pointer to the array so
they can write straightforward code instead of having to go through a
callback.
The text progress and update_tips callbacks are already part of the
struct, which was meant to unify the callback setup, but the download
one was left out.
