I have moved everything over to ioredis, torn out all the callback stuff, and gotten all the tests back to passing! It took a while, but it's pretty much the main hurdle/technical risk for making the 2.0 plan work. That's enough to warrant a WIP branch. I will ping people here when it's less WIP and more ready for review.

Some notes/thoughts:

• Can't wait to refactor a bunch of promise stuff to async/await now that tests are back to green
• I really really like ava, and wow the tests run so fast! 3-4 seconds on my laptop.
• ioredis
  • Their auto-reconnection stuff is great - actually simplified some tests and makes me feel good about trying to test and trust connection re-establishment more
    • keeping an eye on ioredis issue 610 to cover bases/maybe repro potential issues
  • Still lots of little todos to utilize some of their nice features, cluster capabilities, etc
    • also a cool defineCommand capability for lua scripts - could obviate a lot of the load-script-cache stuff we've implemented
  • filed issue 614 on ioredis to resolve a confusion on status/ready detection/timing
• tearing out callbacks
  • Deleted so many instances of if (cb) helpers.asCallback(promise, cb);
  • new rule that your .process handler should just be an async func is great
• Moved promise-callbacks down to a devDependency
  • using several of them in tests so kept whole thing as devDependency
  • implementation code is basically only using waitOn and withTimeout though, so plucked those two into helpers file

Other todos:

• maybe add debug logs, ioredis's debug logs were v helpful at times
• figure out where a percent of test coverage went and get it back
• investigate viability of sharing event subscriber connections, worker connections
• bunch of readme/example updates to expunge callbacks
• redo benchmarks
• probably include some other features in 2.0 for completeness, like pluggable job data serializers and stallLimit

Refs #104