### 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/main/java/org/elasticsearch/search/aggregations/bucket/range/RangeAggregator.java
* artifacts at serialization time as a result of treating float values
         * as double values.
         * (See {@link RangeAggregationBuilder#innerBuild(
         * AggregationContext, ValuesSourceConfig, AggregatorFactory, AggregatorFactories.Builder
         * )})
         */
        public Range(
            final String key,
            final Double from,
            final String fromAsStr,
            final Double to,
            final String toAsStr,
            final DoubleUnaryOperator fixPrecision
        ) {
            this.key = key;
            this.from = from == null ? Double.NEGATIVE_INFINITY : fixPrecision.applyAsDouble(from);
            this.originalFrom = from == null ? Double.NEGATIVE_INFINITY : from;
            this.fromAsStr = fromAsStr;
            this.to = to == null ? Double.POSITIVE_INFINITY : fixPrecision.applyAsDouble(to);
            this.originalTo = to == null ? Double.POSITIVE_INFINITY : to;
            this.toAsStr = toAsStr;
        }
        public Range(String key, Double from, String fromAsStr, Double to, String toAsStr) {
            this(key, from, fromAsStr, to, toAsStr, IDENTITY);
        }
        public Range(String key, Double from, Double to) {
            this(key, from, null, to, null);
        }
        public Range(String key, String from, String to) {
            this(key, null, from, null, to);