Big refactor to make the backend registerable and swappable

[jwoertink]

Ref #88

This pulls in some code from #79

This is a big breaking change:

• redis_ping_interval is renamed to backend_ping_interval
• ping_redis_subscribe is renamed to ping_subscribe_connection
• ping_redis_publish is renamed to ping_publish_connection
• Cable::RedisPinger is renamed to Cable::BackendPinger
• Using ENV["REDIS_URL"] is now ENV["CABLE_BACKEND_URL"]
• CloseRedisFiber didn't seem to be used anywhere so it was removed
• Backends now need to be registered with register "scheme"
• redis is no longer required out of the box

The next few PRs will be removing the redis backend completely and moving it to a separate shard. I'll be making 2 different redis backend shards to allow using either shard without conflicts.

cc. @jgaskins

[jgaskins]

Thank you so much for finding a way to push this through! I remember when we left off before there was some confusion around blocks passed to Habitat setting macros and I never followed up on it.

Would it be possible to throw a Co-authored-by trailer on the commit message for 2ab304e?

Co-authored-by: Jamie Gaskins <jgaskins@hey.com>

[jwoertink]

Sweet! Got some green. @jgaskins added you in there, thanks!

Yeah, I found out Habitat will need a few major refactors in order to support the annotations and block values. For now I just decided since Cable isn't 1.0, let's just break stuff 😂 The current v0.3.1 release is up to date with commits before this PR, so moving forward, I think we can just roll out v0.4.0 with the updated interface.