make it possible to skip the handshake negotiation

[marten-seemann]

This PR makes changes to the upgrader, so we can use it to handle both multiaddrs with and without the security protocol.
Depends on libp2p/go-libp2p-core#215.

For multiaddrs not containing the security protocol:

• Upgrader.SecureMuxer needs to be set (this is the same as it is now). The upgrader will then use multistream to negotiate the security protocol.

For multiaddrs containing the security protocol:

• Upgrader.SecureTransport needs to be set (and Upgrader.SecureMuxer must not be set). In that case, the upgrader will jump into the cryptographic handshake right after having dialed / accepted the net.Conn.

This PR also adds a Upgrader.SecurityProtocol() ma.Protocol method, which returns the security protocol of the SecureTransport. Perhaps a bit surprisingly, this returns the zero-value of ma.Protocol in case a SecureMuxer is set. We could have introduced a valid bool here, but I chose not to do so, as we'll be dropping support for the SecureMuxer soon-ish, and having just a single return value will be nicer then.

[marten-seemann]

Is there any better solution for this? This is... ugly.

[raulk]

What's the plan for exposing multiple handshakes over a single endpoint?

[marten-seemann]

What's the plan for exposing multiple handshakes over a single endpoint?

The idea is that every instance of a Transport supports exactly one cryptographic handshake. That means we’ll have one Transport for TLS/TCP and one for Noise/TCP.
During the transition period, we’ll have one more Transport that uses multistream for handshake negotiation (that’s the one where SecureMuxer is set instead of SecureTransport).