Idea: BloomFilterAggregator can directly rely on a BitSet instead of a BFMonoid

[anish749]

@johnynek I was going through your talk here from Scale by the Bay 2018 about immutable APIs and mutable internals.

I had a very rough idea about the BloomFilterAggregator and how it would look following the design pattern you discussed, I tried it in this gist here

Do you think this would work if we use a version of the cats BitSet which works great when a low number of bits are set?