Batch dialling

[achingbrain]

The libp2p.dial method should support dialling multiple multiaddrs - it currently supports dialing a peer id (in which case multiaddrs are retrieved from the peer store) or a single multiaddr.

Since only a single connection is returned from a dial, the multiaddrs being dialled should be checked to ensure they either all contain the same peer id or that none of them contain a peer id.

If they all have the same peer id we are guaranteed to return a connection to the expected remote peer, if none of them contain a peer id then the user did not expect a connection to a specific peer so we are ok to return a connection to whatever remote peer we managed to connect to first.