Network crate cleanups

[tomaka]

Now that the tests are not in the way anymore (#3016), we can clean up the API surface of the networking crate.

While the diff is quite large, it can easily be reviewed commit by commit.
Most of the changes are very straight-forward and should be uncontroversial.

In terms of logic, I moved some methods from the NetworkService to the NetworkWorker. This made it possible to remove the Arc<Mutex> around the libp2p Swarm.

The network crate now only contains three Arcs and zero Mutex or RwLock, if you exclude the Arcs to the various other components of Substrate (eg. Arc<Client>).