Simplify querier merge code

[yeya24]

What this PR does:

Replaces querier.mergeSeriesSets with storage.NewMergeSeriesSet(result, storage.ChainedSeriesMerge) because basically they do the same thing.

Which issue(s) this PR fixes:
Fixes #

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[yeya24]

I am also thinking about getting rid of this chunkIterFn completely so that we can remove a ton of code and just reuse upstream interfaces. It makes so much easier to support native histograms.

But this breaks the batch iterator we have.

		iterables := make([]chunkenc.Iterable, len(chunks))
		serieses = append(serieses, &storage.SeriesEntry{
			Lset: ls,
			SampleIteratorFn: func(iterator chunkenc.Iterator) chunkenc.Iterator {
				for i, c := range chunks {
					iterables[i] = c.Data.SampleIterable()
				}
				return storage.ChainSampleIteratorFromIterables(iterator, iterables)
			},
		})