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.
Steven Allen
Raúl Kripalani
vyzo