Redis and ps check for mperham/sidekiq

Monitors sidekiq via its (namespaced) redis keys per the schema
used in http://git.io/e-QGLw (lib/sidekiq/api.rb)

Per @mperham in #1150, this schema is stable.

Also does a service check on the existence and non-terminated state
of worker processes with a given tag.

Supersedes, and thereby closes #1150 

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I would suggest there might still be an interesting pattern generally in that PR - but this one is more practical
It's common usage to use the same typename here, you should rename to `SidekiqStat`