Avoid conflict error hotspot in PluginIndexes' Unindex class by using IITreeSets instead of simple ints from the start. Idea taken from ``enfold.fixes``. This optimizes the common case of FieldIndexes being used for a low number of unique values. The edge-case of having a FieldIndex for an unique value per document gets penalized, as it uses many more persistent objects. This really warrants a separate type of index.