Delegate redis connection build to redix

[thiamsantos]

Motivation

Recently I was setting up phoenix_pubsub_redis in our application, and I incurred in a few problems connecting to ElastiCache redis. The most confusing thing was that I was able to connect to redis using Redix.start_link normally, but if I used the same opts with phoenix_pubsub_redis it would fail. In the end the problem was that the option username is been ignored.

Looking in the repository we have a few other open and closed PRs with similar issues.
Updating the code just to add behaviour on the parsing of redis urls or allowing new options that redix supports.

• pass ssl option to redix #38
• Allow socket opts on ssl #43
• 2.x Redix socket_opts config #46
• Allow 'rediss' URL scheme to define whether SSL is enabled #58
• Don't use defaults when :sentinel is set #60

Proposed Solution

I would like to propose delegating the handle of redis connection opts directly to redis,

• so we don't have to maintain a version of the url parsing inside of phoenix_pubsub_redis
• remember to always have a updated version of the opts supported by redix inside the codebase

We could do that by introducing a new option called redis_opts and then pass that option directly to redix, and soft-deprecate the old option (host, password etc).
In order to keep backward compatibility we could continue supporting the existing options, but encourage the use of the new redis_opts.

In addition to that we can start using the url parsing directly from redix. Currently the function Redix.URI.opts_from_uri/1 is private. I plan to open a discussion on redix on the best way to pass this url to redix, so maybe we can remove this responsibility completely from the codebase.

Does the approach makes sense?

[mitchellhenke]

Thanks for the PR, I've incorporated many of the changes here in #76!