KrakenUniq v0.6 with improved preloading of the database

KrakenUniq (and also Kraken) often ran very slow with really big databases. The problem was that --preload didn't truly force to load the DB in memory, so it spends forever (many days) going back and forth to disk. With the fix included in this release, krakenuniq ran in 16 minutes on a database where before it took >100 hours.

Thanks a lot to @alekseyzimin for the development and contribution of the fix and @salzberg for reporting the issue!