[MRG] gather optimizations

[luizirber]

A couple of fixes for gather perf problems

• don't recalculate scaled query minhash everytime we reach a leaf in the SBT
• add a remove_many method to remove hashes from a minhash
• use the remove_many method to remove matches from the query (instead of building a new one). This makes a huge difference for large queries.

I ran master with a large metagenome query, got 4 matches in 17h. This PR takes 17 minutes to find the same matches.

Checklist

• Is it mergeable?
• make test Did it pass the tests?
• make coverage Is the new code covered?
• Did it change the command-line interface? Only additions are allowed
  without a major version increment. Changing file formats also requires a
  major version number increment.
• Was a spellchecker run on the source code and documentation after
  changes were made?

[codecov]

Codecov Report

Merging #615 into master will decrease coverage by 0.06%.
The diff coverage is 96.55%.

@@            Coverage Diff             @@
##           master     #615      +/-   ##
==========================================
- Coverage   89.45%   89.38%   -0.07%     
==========================================
  Files          27       27              
  Lines        4191     4203      +12     
  Branches       37       39       +2     
==========================================
+ Hits         3749     3757       +8     
- Misses        442      446       +4

Impacted Files	Coverage Δ
sourmash/kmer_min_hash.hh	100% <100%> (ø)	⬆️
sourmash/search.py	94.19% <100%> (-0.04%)	⬇️
sourmash/sbtmh.py	85% <90.9%> (-0.3%)	⬇️
sourmash/commands.py	89.08% <0%> (-0.35%)	⬇️

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[ctb]

LGTM!

[luizirber]

I'll add some tests for remove_many before merging

[ctb]

Nice work!