Add LabelQuerier.LabelValuesStream method

[aknuds1]

Add method storage.LabelQuerier.LabelValuesStream that returns an iterator over corresponding label values, instead of a slice.

Preliminary benchmark statistics, for respectively block and head index readers:

benchstat-block.txt
benchstat-head.txt

Depends on #576

Note to self
When the time comes to upstream, first post a proposal issue, to allow for discussion, before making a PR.

TODO:

• Benchmark
• Instead of making a sorted values slice, consider copying the postings matching the matchers, before intersecting them with each label value’s posting list, allowing iterating over values in alphabetical order (thanks, @dimitarvdimitrov)
• Merge tsdb.IndexReader: Add LabelValuesFor/LabelValuesExcluding methods #576
• Extend test cases to cover relevant corners