Support sorting of documents at merge time

[clintongormley]

Documents of different types tend to have different fields, which can lead to problems of sparsity when documents are stored in the order that they were indexed. If we sort documents by type, then missing values (eg in doc values or norms) can be represented very efficiently.

On merge, documents should be sorted by type (assuming there is more than one type). Additionally, for certain use cases, it can make sense to apply a secondary sort (eg on timestamp). This secondary level should be customizable per index, and should probably be dynamically updatable (although it would only have effect on later merges).

Relates to #8870

[clintongormley]

Closing as duplicate of #6720