Consider alphabetical ordering for TOPK with Strings

[rodesai]

The default String comparator we use for comparing strings orders all upper-case characters after lower-case characters. This is likely to be inconsistent with what a user would expect.

[big-andy-coates]

Our TopK style aggregators use reverse comparators, regardless of type. So no only does it put uppercase after lower, it also puts numbers after lower.

Though changing this would break anyone currently using strings with TopK...