Add ability to sort on multiple criteria

[colings86]

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
Replaces #7588

[colings86]

First of the commits is the original from #7588, the second is the fix to the backwards compatibility issues and the intermittent test failures

[jpountz]

This looks good to me. I think we should push this to 1.x. On master it makes sense to not have this isSingleUserCriteria work-around since it will not have to be able to talk to 1.x releases?

[colings86]

Ok, I will push this change and then raise a new PR which removes the workaround on master