Version check in/after handshake

[jbenet]

Right after the secure channel handshake, ipfs connections should handshake protocol versions. Incompatible versions should not talk to each other.

I think the right way is to:

• have a semver for the whole protocol*
• send each other protocol version in the connection handshake
• if major doesn't match, bail. Accept minor and patch differences.

*(it's possible to use per-service versions, but ipfs is not large enough / network alive enough to warrant that yet, we can revisit then)

note on semver major number

For those unfamiliar with semver, don't think of a semver <major> = 2 as ipfs protocol v2. It's really ipfs protocol 1.<major>.<minor>.<patch> in that a 2.x.x.x IMO means a massive rethinking. (think ipv4 -> ipv6)

[whyrusleeping]

Should reference issue #161 for this.

[jbenet]

Addressed in #180.