Add support for constructing the bloom filter on a prefix of the user key. Add a Comparator.PrefixExtractor function for extracting the prefix.

One question here is whether this should be considered a prefix extractor or restricted to user keys which have a version. An alternate api would be to provide Comparator.Split to split a user key into a prefix and a version suffix. This might be useful in the future for segregating historic versions of user keys into a separate storage area.