Added support for cert-only login without user and password

[mihaitodor]

This PR contains the changes proposed in streadway/amqp#121 by
@p-kraszewski together with the changes that @wolf-james requested and some small comment additions for consistency.

Note that I'm not really familiar with this protocol and I haven't looked into setting up a test environment for this setup. People have confirmed that the changes are correct in the original PR, but please let me know if you'd prefer some extra validation and I'll look into it when I'll have some time.

Update: I was able to validate these changes and I documented the steps I followed here: https://github.com/mihaitodor/amqp_tls_go_demo

[Zerpet]

This PR makes sense to me. WDYT @michaelklishin @ChunyiLyu ?

[mihaitodor]

Hey everyone, if possible, could you please let me know if there’s a chance this API addition will be released soon?

[Zerpet]

I'd like to investigate some of the issues we have regarding dead locks before we produce a new release; those seem important. I won't commit to a date, but in a few weeks, depending on the conclusion of those issues, sounds reasonable to produce a new release.

Now that the PR is merged, you could pin your Go dependency to the commit SHA. Not ideal, but at least you'll be able to use external auth.

[mihaitodor]

Awesome, thank you very much @Zerpet! Indeed, deadlocks don't sound good. I'll have a look too.