Previously `libp2p-swarm` required a `Transport` to be `Clone`. Methods
on `Transport`, e.g. `Transport::dial` would take ownership, requiring
e.g. a `Clone::clone` before calling `Transport::dial`.

The requirement of `Transport` to be `Clone` is no longer needed in
`libp2p-swarm`. E.g.  concurrent dialing can be done without a clone per
dial.

This commit removes the requirement of `Clone` for `Transport` in
`libp2p-swarm`. As a follow-up methods on `Transport` no longer take
ownership, but instead a mutable reference (`&mut self`).

On the one hand this simplifies `libp2p-swarm`, on the other it
simplifies implementations of `Transport`.