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fixes #939

Routing is only required for autorelay when no static relays are configured.

and rename DefaultRelays option to DefaultStaticRelays.

The new `Filters` option allows users to take ownership of the `Filters` struct, and is suitable for cases where the addresses you want to accept/deny change dynamically.

Instead of using a global variable.

This also:

* Adds an option to the identify service to set the user agent.
* Removes the ability to pass an identify service to NewHost as any reasonable
  Identify service already needs to be constructed with an instance of the host.

So, in go-ipfs at least, we *don't* pass the listen addresses into the libp2p
constructor. Instead, we specify them later.

This option allows us to tell libp2p to not listen by default.

Also, make the libp2p constructor fully useful. There should now be no need to
manually construct a swarm/host.