[MRG] Refactor the gather_databases function to use MinHash API over Python set API.

[ctb]

Much simplification and (potentially?) optimization of gather_databases function, based on various improvements in #1370 and #1475.

[codecov]

Codecov Report

Merging #1512 (a9ca357) into latest (91603d4) will increase coverage by 5.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           latest    #1512      +/-   ##
==========================================
+ Coverage   90.25%   95.27%   +5.02%     
==========================================
  Files         126       99      -27     
  Lines       21117    17404    -3713     
  Branches     1591     1589       -2     
==========================================
- Hits        19059    16582    -2477     
+ Misses       1829      593    -1236     
  Partials      229      229              

Flag	Coverage Δ
python	95.27% <100.00%> (-0.01%)	⬇️
rust	?

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Impacted Files	Coverage Δ
src/sourmash/commands.py	86.27% <100.00%> (-0.02%)	⬇️
src/sourmash/search.py	96.24% <100.00%> (-0.21%)	⬇️
src/core/src/from.rs
src/core/src/index/search.rs
src/core/src/sketch/nodegraph.rs
src/core/src/sketch/hyperloglog/mod.rs
src/core/src/ffi/mod.rs
src/core/src/encodings.rs
src/core/src/ffi/cmd/compute.rs
src/core/src/index/mod.rs
... and 19 more

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[ctb]

Ready for review and merge! (tests passed, just updated.)