Stop relying on a `Clone`able `NetworkGraph` ref in `DefaultRouter`

Stop relying on a `Clone`able `NetworkGraph` ref in `DefaultRouter`

While there's not really much harm in requiring a `Clone`able
reference (they almost always are), it does make our bindings
struggle a bit as they don't support multi-trait bounds (as it
would require synthesizing a new C trait, which the bindings don't
do automatically). Luckily, there's really no reason for it, and we
can just call the `DefaultMessageRouter` directly when we want to
route a message.

We've carried this patch for a while on the bindings branch, but
there's not a strong reason it can't go upstream.