Feat/expose connection handling flow in dial

[dryajov]

@diasdavid @dignifiedquire

I need the ability to pass a raw connection to the swarm and have it encrypted and muxed, this is required for onion dialing, since when I get a conn from one relay to another relay, I have no way to upgrade that with encryption and muxing (handshake).

Here is a typical flow:

• having a chained multiaddr like this /ipfs/QmRealy1/p2p-circuit/ipfs/QmRelay2/p2p-circuit/ipfs/QmDest
  • I need to dial into /ipfs/QmRealy1/ and hand it the /ipfs/QmRelay2 address as a dest
    • at this point, a relay1 <=> relay2 circuit exists, but since it hasn't been returned to the swarm it has not gone through muxing and encryption. Next time I read from that connection, I get a multristream header sent from QmRelay2 that is the start of the regular p2p handshake, for the relay1 <=> relay2 connection.

Exposing the the dial/handshake flow would allow me to manually send the relay1 <=> relay2 to the swarm and have it go through the handshake. The other option would be to reimplement the handshake in circuit, but I don't think we want to do that :)

[daviddias]

Is this still being used? @dryajov If yes, could you coalesce this PR with #204 for me to review?

Thank you!

[dryajov]

This is already part of #181 and can be safely closed.