### Description

Our most common aggregations should be optimized for single-valued fields. In general, our aggregations look like this:

```java
    @Override
    public LeafBucketCollector getLeafCollector(LeafReaderContext ctx, LeafBucketCollector sub) throws IOException {
        if (valuesSource == null) {
            return LeafBucketCollector.NO_OP_COLLECTOR;
        }
        SortedNumericDocValues values = valuesSource.longValues(ctx);
        return new LeafBucketCollectorBase(sub, values) {
            @Override
            public void collect(int doc, long owningBucketOrd) throws IOException {
                if (values.advanceExact(doc)) {
                    int valuesCount = values.docValueCount();
                    for (int i = 0; i < valuesCount; ++i) {
                        long value = values.nextValue();
                        // do something with value
                    }
                }
            }
        };
    }
```

The idea consists of optimizing for single-valued fields by unwrapping the single-valued doc-value iterator:

```java
    @Override
    public LeafBucketCollector getLeafCollector(LeafReaderContext ctx, LeafBucketCollector sub) throws IOException {
        if (valuesSource == null) {
            return LeafBucketCollector.NO_OP_COLLECTOR;
        }
        SortedNumericDocValues values = valuesSource.longValues(ctx);
        NumericDocValues singleValues = DocValues.unwrapSingleton(values); // or FieldData.unwrapSingleton for doubles
        if (singleValues != null) {
            return new LeafBucketCollectorBase(sub, values) {
                @Override
                public void collect(int doc, long owningBucketOrd) throws IOException {
                    if (singleValues.advanceExact(doc)) {
                        long value = singleValues.longValue();
                        // do something with value
                    }
                }
            };
        } else {
            return new LeafBucketCollectorBase(sub, values) {
                @Override
                public void collect(int doc, long owningBucketOrd) throws IOException {
                    if (values.advanceExact(doc)) {
                        int valuesCount = values.docValueCount();
                        for (int i = 0; i < valuesCount; ++i) {
                            long value = values.nextValue();
                            // do something with value
                        }
                    }
                }
            };
        }
    }
```

The `terms` aggregation already does this, and e.g. `min` and `max` aggregations use similar tricks that also help optimize the single-valued case. But `date_histogram`, which is one of our most common aggregations, doesn't. Let's specialize the single-value case for date histograms too?
server/src/test/java/org/elasticsearch/search/aggregations/bucket/histogram/DateHistogramAggregatorTests.java
Arrays.asList(
                "2017-02-01T09:02:00.000Z",
                "2017-02-01T09:35:00.000Z",
                "2017-02-01T10:15:00.000Z",
                "2017-02-01T13:06:00.000Z",
                "2017-02-01T14:04:00.000Z",
                "2017-02-01T14:05:00.000Z",
                "2017-02-01T15:59:00.000Z",
                "2017-02-01T16:06:00.000Z",
                "2017-02-01T16:48:00.000Z",
                "2017-02-01T16:59:00.000Z"
            ),
            aggregation -> aggregation.calendarInterval(DateHistogramInterval.HOUR).field(AGGREGABLE_DATE).minDocCount(1L),
            histogram -> {
                List<? extends Histogram.Bucket> buckets = histogram.getBuckets();
                assertEquals(6, buckets.size());
                Histogram.Bucket bucket = buckets.get(0);
                assertEquals("2017-02-01T09:00:00.000Z", bucket.getKeyAsString());
                assertEquals(2, bucket.getDocCount());
                bucket = buckets.get(1);
                assertEquals("2017-02-01T10:00:00.000Z", bucket.getKeyAsString());
                assertEquals(1, bucket.getDocCount());
                bucket = buckets.get(2);
                assertEquals("2017-02-01T13:00:00.000Z", bucket.getKeyAsString());
                assertEquals(1, bucket.getDocCount());
                bucket = buckets.get(3);
                assertEquals("2017-02-01T14:00:00.000Z", bucket.getKeyAsString());
                assertEquals(2, bucket.getDocCount());
                bucket = buckets.get(4);
                assertEquals("2017-02-01T15:00:00.000Z", bucket.getKeyAsString());