### 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
import org.elasticsearch.index.mapper.KeywordFieldMapper;
import org.elasticsearch.index.mapper.LongScriptFieldType;
import org.elasticsearch.index.mapper.MappedFieldType;
import org.elasticsearch.index.mapper.NumberFieldMapper;
import org.elasticsearch.index.mapper.NumberFieldMapper.NumberType;
import org.elasticsearch.script.LongFieldScript;
import org.elasticsearch.script.Script;
import org.elasticsearch.script.StringFieldScript;
import org.elasticsearch.search.aggregations.Aggregator;
import org.elasticsearch.search.aggregations.AggregatorTestCase;
import org.elasticsearch.search.aggregations.CardinalityUpperBound;
import org.elasticsearch.search.aggregations.support.AggregationInspectionHelper;
import org.elasticsearch.search.lookup.SearchLookup;
import org.elasticsearch.search.runtime.StringScriptFieldTermQuery;
import java.io.IOException;
import java.time.ZoneOffset;
import java.time.ZonedDateTime;
import java.util.Collections;
import java.util.List;
import java.util.Map;
import java.util.concurrent.TimeUnit;
import java.util.function.Consumer;
import static java.util.Collections.singleton;
import static java.util.stream.Collectors.toList;
import static org.elasticsearch.test.MapMatcher.assertMap;
import static org.elasticsearch.test.MapMatcher.matchesMap;
import static org.hamcrest.Matchers.closeTo;
import static org.hamcrest.Matchers.equalTo;
import static org.hamcrest.Matchers.hasSize;
public class RangeAggregatorTests extends AggregatorTestCase {