Decouple ConfigDB & Ruler

[jtlisi]

This is the first of several PRs that will decouple the ruler from the configdb and allow for the updated API and flexible storage options as outlined in this design document

This PR is a subset of the changes in #1513, and is meant to break the referenced PR into smaller more manageable components

• Decouple the scheduler from the polling mechanism used to query the configdb
• Update the scheduler to use a user-based context to allow for cancellations after updates or deletions
• Pass a prometheus register with the users ID to the prometheus rule group to get user level prom eval metrics

Signed-off-by: Jacob Lisi jacob.t.lisi@gmail.com