Relay for go-ipfs <=> browser connectivity

[flyingzumwalt]

The work:

• Bridging connectivity gaps: Relay to nodes that don't support websockets or webrtc

The prespec is here: https://github.com/libp2p/specs/blob/d76e2bc5a594c7aa22427f81fa040935ffc6e80b/relay/README.md -- open spec+impl questions and issues in the libp2p/specs repo.

• implement js-libp2p-circuit
• wire up with js-ipfs
• implement go-libp2p-circuit
• wire up with go-ipfs
• deploy public default circuit relay nodes

[flyingzumwalt]

Probably won't get done in this sprint because it needs relay.

@dryajov has started an implementation of js-libp2p-circuit: https://github.com/libp2p/js-libp2p-circuit

Work on circuit relay in go-ipfs has started too: https://github.com/libp2p/go-libp2p-circuit