Disabling of individual jobs and improving the Class Name => Redis Namespace mapping

As a first step to the ability to enable/disable repeating sidetiq jobs, I've added a new check in handler that prevents the repeating job from firing if a "disabled" redis key is set.  Also created API methods to enable and disable all jobs.

I've also improve the class name to redis namespace mapping.  I have some of my worker classes namespaced and the default mapping was causing "ugly" redis namespacing.  Example
```
sidekiq:sidetiq::Worker::Repeat::TriggerDiscovery::locked
```
With my new approach the same key shows up in lowercase underscore and no double colons, as follows
```
sidekiq:sidetiq:worker:repeat:trigger_discovery:locked
```
If you're open to this PR, let me know if you'd like something changed or modified.