[MRG] Fix sourmash prefetch to work when db scaled is larger than query scaled

[ctb]

This PR fixes a problem where sourmash prefetch fails to properly downsample the query sketch as needed.

TODO:

• do we need to check against other database types?
• fix assert in re threshold calculation; allow skip? maybe test this more.
• notify user of scaled value for final summary sketches
• make sure that output sketches are at appropriate scaled

[codecov]

Codecov Report

Merging #1870 (f3b1a7a) into latest (234df70) will increase coverage by 7.98%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           latest    #1870      +/-   ##
==========================================
+ Coverage   82.37%   90.35%   +7.98%     
==========================================
  Files         119       89      -30     
  Lines       12935     8742    -4193     
  Branches     1727     1729       +2     
==========================================
- Hits        10655     7899    -2756     
+ Misses       2016      579    -1437     
  Partials      264      264              

Flag	Coverage Δ
python	90.35% <100.00%> (+<0.01%)	⬆️
rust	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/sourmash/commands.py	88.43% <100.00%> (+0.09%)	⬆️
src/sourmash/search.py	97.00% <100.00%> (+0.02%)	⬆️
src/core/tests/minhash.rs
src/core/src/sketch/minhash.rs
src/core/src/ffi/mod.rs
src/core/src/sketch/hyperloglog/estimators.rs
src/core/src/ffi/cmd/compute.rs
src/core/src/index/search.rs
src/core/src/ffi/nodegraph.rs
src/core/src/index/mod.rs
... and 22 more

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[bluegenes]

a couple minor things, otherwise lgtm!