Free AggregatorFactory implementors from boilerplate in getMergingFactory()

Currently AggregatorFactory implementors should write code like the following themselves:
```java
if (other.getName().equals(this.getName()) && other.getClass() == getClass()) {
...
} else {
throw new AggregatorFactoryNotMergeableException(...);
}
```
or
```java
if (other.getName().equals(this.getName()) && other instanceof MyBaseAggregatorFactory) {
...
} else {
throw new AggregatorFactoryNotMergeableException(...);
}
```

if `MyBaseAggregatorFactory` is extended by other aggregator factory classes.

It could be replaced by e. g. by adding a generic parameter `BaseMergeAggregatorFactory` to `AggregatorFactory` class; adding a method `Class getBaseMergeAggregatorFactory()` method; and changing the signature of `getMergeFactory()` to accept `BaseMergeAggregatorFactory` as the parameter.

Start by locating AggregatorFactory, getMergingFactory(), and existing AggregatorFactory implementors that repeat the name and type checks. Review how AggregatorFactoryNotMergeableException is used; done means the shared merge contract removes that boilerplate while preserving the existing mergeability behavior.

Written by the indexing model from the issue text.