Debug impl for MultiplexedConnection & Pipeline

[elpiel]

Hello,
First of all, thanks for the amazing work on the redis crate!
I'm trying to use the tracing crate for an async application with tokio & tokio-console, however, I'm unable to use the #[instrument] attribute on async function as there is no Debug impl for MultiplexedConnection & Pipeline

Would you consider such addition to the code?
aio::Pipeline seems the most difficult part to implement. What information and other considerations should be taken into account for implementing Debug for Pipeline?

[djc]

Yes, I would be open to having Debug implementations for these. I don't have much of an idea about what should be in them, please submit a PR using your best judgement and I'll review from there.

[elpiel]

Ok, let me take a look and see what I can impl