Added backoff to fetcher worker to partially address #746

[mhutchinson]

This is a huge improvement from retying without any backoff. However, this could still benefit from:

• being tunable
• supporting hard failing if requests can't be completed within the backoff configuration
• resetting the backoff if requests start succeeding faster

[codecov]

Codecov Report

Merging #747 (4be0e96) into master (7710282) will increase coverage by 0.05%.
The diff coverage is 90.90%.

@@            Coverage Diff             @@
##           master     #747      +/-   ##
==========================================
+ Coverage   72.72%   72.78%   +0.05%     
==========================================
  Files          81       81              
  Lines        7887     7893       +6     
==========================================
+ Hits         5736     5745       +9     
+ Misses       1717     1715       -2     
+ Partials      434      433       -1     

Impacted Files	Coverage Δ
scanner/fetcher.go	60.18% <90.90%> (+5.28%)	⬆️

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[mhutchinson]

Latest commit addresses "resetting the backoff if requests start succeeding faster" from the initial description. This is still incrementally much better (and with no downsides compared to the original?), but not ideal.