[experiment] use Mint for HTTP

[paulswartz]

Mint is a new HTTP library which doesn't spin up new processes for each request. Instead, it lets the user manage the process. Since Concentrate already has a separate process for each fetch (since we're using separate stages), I thought this might map nicely to the existing model.

[codecov]

Codecov Report

Merging #82 (3cb44bf) into master (6e86872) will decrease coverage by 0.89%.
The diff coverage is 88.80%.

@@            Coverage Diff             @@
##           master      #82      +/-   ##
==========================================
- Coverage   97.10%   96.20%   -0.90%     
==========================================
  Files          58       59       +1     
  Lines        1035     1160     +125     
==========================================
+ Hits         1005     1116     +111     
- Misses         30       44      +14     

Impacted Files	Coverage Δ
lib/concentrate/producer/mint.ex	88.80% <88.80%> (ø)
lib/concentrate/trip_descriptor.ex
...centrate/group_filter/trip_descriptor_timestamp.ex
.../concentrate/group_filter/trip_update_timestamp.ex	100.00% <0.00%> (ø)
lib/concentrate/trip_update.ex	93.33% <0.00%> (ø)

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