Make RoutingMessageHandler a generic Deref instead of an Arc

Make RoutingMessageHandler a generic Deref instead of an Arc

We also update to use single idents when referencing the Deref=*
types since the automated code generator is pretty braindead.

This also moves some test utils out of peer_handler.rs and into
util::test_utils to standardize things a little bit, which we need
to concretize the PeerHandler types used in testing.