Design new identify protocols

[tomaka]

The current identify protocol is not great, so let's design new ones.

I think we need three protocols:

• One that only reports the observed address, for NAT traversal.
• One that reports information about the node, including its name. That could be done in a generic way, in other words fields wouldn't be hard-coded.
• One that only reports the public key of the node.

All that is needed for each protocol is something similar to this: https://github.com/libp2p/rust-libp2p/blob/master/protocols/identify/src/protocol.rs
No need for new transports.

cc paritytech/substrate#398

[romanb]

Closing as a result of #1238.