Revise the relay protocol

[raulk]

Carried over from libp2p/go-libp2p#578 (comment):

A relay client can be a consumer of a relay service, or a target of a relayed connection. Currently, all of these flow over the same thick protocol. I really have a hard time wrapping my head around this, and so do our users.

I'd like to segregate this protocol if possible into at least two:

• /libp2p/circuit/relay/hop/v0.0.0: exposed only by relay nodes. Used inside the initiator -> relay connection.
• /libp2p/circuit/relay/target/v0.0.0: exposed by clients accepting relayed connections. Used in the relay -> target connection.

This would allow for better implicit observability too. Would've helped a lot in the incident we're analysing.

[Stebalien]

libp2p/specs#105

[raulk]

Continuing discussion in libp2p/specs#105.