Allow Querier and Query Frontend to run in the same process

[trevorwhitney]

Is your feature request related to a problem? Please describe.
When running a single-binary instance of Loki, if both the querier and query frontend targets are enabled, then they register the same routes and step on each other.

Describe the solution you'd like
Implement the solution from cortex that uses an internal query server that is optionally registered publicly when only one target is registered, or is referenced internally when both are registered.

Describe alternatives you've considered
Haven't really though of many alternative since this is already done in cortex and should be pretty easy to bring over.