Terms Aggregation: Handle a list of order options

[mtraynham]

It would be nice if you could use a list of ordering options instead of a single order. This is useful for when you need a secondary sort because the first one is of low cardinality.

Doesn't seem too hard to add after looking at the code. InternalOrder could be extended with with a class called InternalOrderChain that can take a list of InternalOrder objects in it's constructor. The comparator could use Guava's Ordering.compound:

    protected Comparator<Bucket> comparator(final Aggregator aggregator) {
        return Ordering.compound(FluentIterable
                .from(orders)
                .transform(new Function<InternalOrder, Comparator<? super Bucket>>() {
                    @Override public Comparator<? super Bucket> apply(InternalOrder input) {
                        return input.comparator(aggregator);
                    }
                })
                .toList()
        );
    }

The only thing that's a bit strange is the InternalOrder.Streams handling and any check to see if the InternalOrder is an Aggregation...

[mtraynham]

Closing in favor of #6917