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It's impossible to cancel this correctly. This patch also tries to avoid
goroutines whenever possible.

This applies orders hierarchically. Unfortunately, the interfaces had to be
changed significantly to get this to work.

For example `[OrderByValue{}, SortByKey{}]` *used* to re-sort by key, throwing
away the value sort. Now, this acts like a normal database and `SortByKey` will
only sort within equivalent values.

None of our actual datastores support any other types and the cast noise gets
really annoying after a while.