[MRG] Avoid calling node.data

[luizirber]

While doing profiling on sourmash gather I found out this line is where we spend almost all the runtime:
matches = sum(1 for value in hashes if node.data.get(value))
This makes sense, since it's the part where we check if the hashes are in the internal node Bloom Filter.

Turns out doing node.data takes some time, so replacing the line with

get = node.data.get
matches = sum(1 for value in hashes if get(value))

gives us ~40% reduced runtime. Yay!

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?

Ready for review and merge @ctb

[codecov-io]

Codecov Report

Merging #516 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #516      +/-   ##
==========================================
+ Coverage   90.75%   90.75%   +<.01%     
==========================================
  Files          33       33              
  Lines        5007     5010       +3     
  Branches       36       36              
==========================================
+ Hits         4544     4547       +3     
  Misses        463      463

Impacted Files	Coverage Δ
sourmash/sbtmh.py	86.86% <100%> (+0.29%)	⬆️

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