### 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/range/RangeAggregatorTests.java
}, range -> fail("Should have thrown exception"), fieldType));
    }
    public void testUnmappedWithBadMissingField() {
        RangeAggregationBuilder aggregationBuilder = new RangeAggregationBuilder("range").field("does_not_exist")
            .addRange(-2d, 5d)
            .missing("bogus");
        MappedFieldType fieldType = new NumberFieldMapper.NumberFieldType(NUMBER_FIELD_NAME, NumberFieldMapper.NumberType.INTEGER);
        expectThrows(NumberFormatException.class, () -> testCase(aggregationBuilder, new MatchAllDocsQuery(), iw -> {
            iw.addDocument(singleton(new NumericDocValuesField(NUMBER_FIELD_NAME, 7)));
            iw.addDocument(singleton(new NumericDocValuesField(NUMBER_FIELD_NAME, 1)));
        }, range -> fail("Should have thrown exception"), fieldType));
    }
    public void testSubAggCollectsFromSingleBucketIfOneRange() throws IOException {
        RangeAggregationBuilder aggregationBuilder = new RangeAggregationBuilder("test").field(NUMBER_FIELD_NAME)
            .addRange(0d, 10d)
            .subAggregation(aggCardinalityUpperBound("c"));
        simpleTestCase(aggregationBuilder, new MatchAllDocsQuery(), range -> {
            List<? extends InternalRange.Bucket> ranges = range.getBuckets();
            InternalAggCardinalityUpperBound pc = ranges.get(0).getAggregations().get("c");
            assertThat(pc.cardinality(), equalTo(CardinalityUpperBound.ONE));
        });
    }
    public void testSubAggCollectsFromManyBucketsIfManyRanges() throws IOException {
        RangeAggregationBuilder aggregationBuilder = new RangeAggregationBuilder("test").field(NUMBER_FIELD_NAME)
            .addRange(0d, 10d)
            .addRange(10d, 100d)
            .subAggregation(aggCardinalityUpperBound("c"));
        simpleTestCase(aggregationBuilder, new MatchAllDocsQuery(), range -> {
            List<? extends InternalRange.Bucket> ranges = range.getBuckets();
            InternalAggCardinalityUpperBound pc = ranges.get(0).getAggregations().get("c");