Making IPFS Client thread-safe. Using cURL Multi API.

[melroy89]

• Using LibcURL Multi API
• Allowing to abort IFPS API calls
• Fix nlohmann json hpp file (during make install installation)
• Improved readme/docs
• Improve doxygen & CI
• Update go-ipfs binary during testing
• Upgrade to mac os x 11 (with newer xcode sdk)
• Adding a C++ multi-threading example!! 👍🏽
• Threading test-case added as well 🏅

During review, focus on changes to files:

• src/http/transport-curl.cc file.
• src/client.cc
• test/test_threading.cc
• See the example: examples/threading_example.cc 💯

Fixes #54

[codecov-commenter]

Codecov Report

Merging #57 (861db76) into master (8e56921) will decrease coverage by 3.34%.
The diff coverage is 29.85%.

@@            Coverage Diff             @@
##           master      #57      +/-   ##
==========================================
- Coverage   56.92%   53.58%   -3.35%     
==========================================
  Files           2        2              
  Lines         390      405      +15     
  Branches      186      202      +16     
==========================================
- Hits          222      217       -5     
- Misses          3       11       +8     
- Partials      165      177      +12     

Impacted Files	Coverage Δ
src/client.cc	58.18% <25.00%> (-0.13%)	⬇️
src/http/transport-curl.cc	43.84% <30.15%> (-9.94%)	⬇️

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[melroy89]

LGTM! @vasild I will merge when everything is green.