Better handle laddrs in Listen

[Stebalien]

1. Listen("/p2p-circuit") should behave as it does today.
2. Listen("/ipfs/QmOther/p2p-circuit") should dial QmOther, keep the connection open (and fail to listen if we can't dial them), and check if they support relaying.
3. Listen should only work if: (1) we understand all the protocols in the local addr and (2) the mutliaddr ends with /p2p-circuit. Currently, we just check if it contains /p2p-circuit and accept it.
4. We should use calls to Listen to signal that we accept relayed connections (and from where).

[Stebalien]

Example for point 4:

• Listen("/p2p-circuit") returns a listener that listens for all inbound circuit relay connections.
• Listen("/ipfs/QmRelay/p2p-circuit") returns a listener that only accepts relayed connections from QmRelay.

This is kind of pedantic but it allows users select which peers they want to use as relays (may be useful in cases where some links are cheaper than others).

[vyzo]

I think point (2) is the really important one, as it ensures we have a connection open to the relay.

[Stebalien]

@vyzo given that we've now enabled relay by default in go-libp2p, I'd like to fix this (at least point 3) ASAP so we can remove this dance.

[Stebalien]

That is, we should:

1. Add a default listen addr of /p2p-circuit.
2. Keep the relay enabled (for dialing) even if we run NoListenAddrs.

[vyzo]

That's fine by me.

[raulk]

@vyzo wondering if we made progress with this? I recall seeing related changes come through.

[vyzo]

no progress to be reported.