Fix for MONDRIAN-1377: NegativeArraySizeException and OOME observed in SegmentBuilder.rollup().  Dense segments were formerly being created when sparse was required.  Check for sparsity now factors in the both size of the axes and cellset.
    Also a minor fix to Util.intersection() in cases where sets are not ArraySortedSet.
Merged from f546579de75fb327ff024cfc632cf80fd6164bae