[WIP] Faster indexing

[luizirber]

• Split nodes into internal and leaves
• allow adding leaves without updating internal nodes (trigger it later)

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-io]

Codecov Report

Merging #456 into master will decrease coverage by 0.68%.
The diff coverage is 82.28%.

@@            Coverage Diff             @@
##           master     #456      +/-   ##
==========================================
- Coverage   89.26%   88.58%   -0.69%     
==========================================
  Files          27       27              
  Lines        4238     4414     +176     
  Branches       39       39              
==========================================
+ Hits         3783     3910     +127     
- Misses        455      504      +49

Impacted Files	Coverage Δ
sourmash/__main__.py	96% <ø> (ø)	⬆️
sourmash/commands.py	85.66% <10.25%> (-3.43%)	⬇️
sourmash/sbt_storage.py	87.87% <90.9%> (+2.48%)	⬆️
sourmash/sbt.py	86.22% <94.57%> (+0.51%)	⬆️

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