This commit sets the copy-on-write feature of the SeriesIDSets, such
that we can make immutable clones of underlying bitmaps efficiently. If
the original bitmap is modified then a copy will be made, which won't
affect the clone.
This commit ensures that cached bitset results at the Index level are
updated whenever new series ids are created that would belong in those
bitsets.
For example, if we have a cached bitset for the tuple {mem, region,
west}, and we add the series mem,host=prod,region=west then we would
update the cached bitset for {mem, region, west} with the series id of
the newly written series.
Edd Robinson