Cortex queryable is incompatible with latest Prometheus

[bboreham]

I had to revert #763 because the querier started returning blank for all queries.

querier/main.go creates two structs with a flag saying one is for metadata-only and one is for queries.

@juliusv says:

the web API took an engine (which was only used for full queries) and a querier (which was only used for metadata queries)
now the engine only takes a queryable and also creates queries based on it, while the engine doesn't know of a queryable upon construction

So we need to either change upstream Prometheus to tell us what kind of result is needed, or figure out some other way to differentiate within Cortex.

[bboreham]

I believe we can differentiate based on whether the SelectParams passed to Select is nil. But this is somewhat fragile, relying on the particular implementation of Prometheus. Then again, that was what we were doing before.

[bboreham]

Fixed in #802