Implement getValueRangeAtIntervalOp for faster range queries.

This also short-circuits optimize() for now, since it is complex to implement
for the new operator, and ops generated by the query layer already fulfill the
needed invariants. We should still investigate later whether to completely
delete operator optimization code or extend it to support
getValueRangeAtIntervalOp operators.
Merge pull request #316 from prometheus/refactor/range-at-interval-op

Implement getValueRangeAtIntervalOp for faster range queries.