Continue reducing the false positive in bwand_and of zambezi position index.
Continue reducing the false positive in bwand_and of zambezi position index.

The default value for the number of hash is increased from 3 to 6, it
could reduce the false position from 0.5% to 0.09%, while its
calculation cost would be doubled.

pombredanne/izenelib,pombredanne/izenelib,izenecloud/izenelib,pombredanne/izenelib,izenecloud/izenelib,pombredanne/izenelib,izenecloud/izenelib,izenecloud/izenelib