refactor, test, and benchmark different scaled factors across the plugin

[ctb]

#504 fixes a bunch of issues where scaled was not being set properly, ref sourmash-bio/sourmash#3384. This led to inconsistent gather results per #505.

once we reach a good resting point with the sourmash decisions in sourmash-bio/sourmash#3384 and maybe sourmash-bio/sourmash#3355 and maybe #501 we should refactor the code, re-evaluate the tests, and benchmark with different scaled factors for both query and db; in large part this is worth doing because a large part of the point of being able to decrease scaled is that it should lead to much faster searches.