Bridge Multiple Druid Clusters

This pull request introduces the following:

- changes to the rules such that a single rule may apply to multiple tiers
- introduces 'forever' rules such that a rule may not always need a notion of "time" associated with it
- introduces a 'bridge' node to encapsulate the idea that one entire Druid cluster can appear to be a single node to another Druid cluster
- all queryable nodes can now announce their priority and the broker can select which nodes to query based on priority, this is for replication of segments across tiers
- this PR is backwards compatible
- tests
- docs to be added once things are verified to be stable
- GUI still needs some work