[EXP] some experiments in speeding up CounterGather

[ctb]

Note: PR into #2123.

Digging into #1771 more.

Things to try -

• screen out identical matches
• try out an in-memory / on-disk SqliteIndex underneath counter?
• apply threshold to entire counter class, rather than on a per-peek basis
• try removing hashes in CounterGather

[codecov]

Codecov Report

Merging #2113 (a236e7a) into latest (526f785) will increase coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           latest    #2113      +/-   ##
==========================================
+ Coverage   91.61%   91.68%   +0.07%     
==========================================
  Files          99       99              
  Lines       11017    11022       +5     
  Branches     2167     2167              
==========================================
+ Hits        10093    10106      +13     
+ Misses        621      612       -9     
- Partials      303      304       +1     

Flag	Coverage Δ
python	91.68% <100.00%> (+0.07%)	⬆️

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Impacted Files	Coverage Δ
src/sourmash/index/__init__.py	96.36% <100.00%> (-0.36%)	⬇️
src/sourmash/sbt_storage.py	89.15% <0.00%> (+3.38%)	⬆️

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[ctb]

closing - see #2145.