[WIP] Add a read-only IPFS client

[luizirber]

This brings in (or should be merged into) #456

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 #557 into refactor/faster_indexing will increase coverage by 0.15%.
The diff coverage is 63.63%.

@@                     Coverage Diff                      @@
##           refactor/faster_indexing     #557      +/-   ##
============================================================
+ Coverage                     88.42%   88.58%   +0.15%     
============================================================
  Files                            27       27              
  Lines                          4329     4414      +85     
  Branches                         39       39              
============================================================
+ Hits                           3828     3910      +82     
- Misses                          501      504       +3

Impacted Files	Coverage Δ
sourmash/__main__.py	96% <ø> (ø)	⬆️
sourmash/sbt.py	86.22% <100%> (+5.29%)	⬆️
sourmash/commands.py	85.66% <7.89%> (-3.43%)	⬇️
sourmash/sbt_storage.py	87.87% <90.9%> (+1.51%)	⬆️
sourmash/lca/command_index.py	90.74% <0%> (+0.44%)	⬆️
sourmash/lca/lca_utils.py	96.03% <0%> (+0.79%)	⬆️
sourmash/signature_json.py	96.87% <0%> (+1.25%)	⬆️

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