[TESTS] Add explicit mappings to IndexAliasesTests.testSearchingFilteringAliasesSingleIndex

This makes sure that all shards know about the `_uid` field.
Aggregations: Adds ability to sort on multiple criteria

The terms aggregation can now support sorting on multiple criteria by replacing the sort object with an array or sort object whose order signifies the priority of the sort. The existing syntax for sorting on a single criteria also still works.

Contributes to #6917