Implement strict version checking

[Sean-Der]

We should check the version of the remote we are communicating with (and throw an error) right now we depend on the other side blowing up if they are v1.0

[at-wat]

https://tools.ietf.org/html/rfc6347#section-4.2.1

The server_version field has the same syntax as in TLS. However, in order to avoid the requirement to do version negotiation in the initial handshake, DTLS 1.2 server implementations SHOULD use DTLS version 1.0 regardless of the version of TLS that is expected to be negotiated. DTLS 1.2 and 1.0 clients MUST use the version solely to indicate packet formatting (which is the same in both DTLS 1.2 and 1.0) and not as part of version negotiation. In particular, DTLS 1.2 clients MUST NOT assume that because the server uses version 1.0 in the HelloVerifyRequest that the server is not DTLS 1.2 or that it will eventually negotiate DTLS 1.0 rather than DTLS 1.2.

RFC says that peers must not strictly check the version in some of the steps.