CascadingSelectSearcher does not implement SortableCustomFieldSearcher and is really inefficent (cpu and memory)

Currently when you search by a cascading select field it is slow and inefficient. This is because CascadingSelectSearcher does not implement SortableCustomFieldSearcher. We were aware of this, so there could be a reason why it doesn't... this has the effect of reading every customfield value for every issue in the search into memory. Not good.

This caused a big customer to run out of memory.