Modified retry package to use backoff package

[kristinapathak]

Fixes https://github.com/xmidt-org/codex-db/issues/20.

Most important differences:

• Instead of a number of retries, we have a maximum amount of time we want to spend trying to insert the records. This means things that fail fast will retry more than things that fail slowly (such as due to timeouts).
• Now we have jitter, so if many inserts happen at the same time and are failing, the load will be better spread out over time.

Still need to:

• [x] add validation of config values back
• [x] review comments and ensure they are accurate
• [x] fix unit tests

[codecov-io]

Codecov Report

Merging #21 into master will decrease coverage by 5.52%. The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #21      +/-   ##
=========================================
- Coverage   57.42%   51.9%   -5.53%     
=========================================
  Files          15      15              
  Lines        1050     921     -129     
=========================================
- Hits          603     478     -125     
+ Misses        445     441       -4     
  Partials        2       2

Impacted Files	Coverage Δ
retry/retry.go	100% <100%> (+2.4%)	:arrow_up:

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