Don't poll S3 unless we have reason to think there are new files available

[ponyisi]

Keep the minio polling interval but do nothing if the current transform status does not indicate more files than the last time we looked in S3.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.59%. Comparing base (847d28a) to head (5ac484b).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #553   +/-   ##
=======================================
  Coverage   92.59%   92.59%           
=======================================
  Files          29       29           
  Lines        1823     1824    +1     
=======================================
+ Hits         1688     1689    +1     
  Misses        135      135           

Flag	Coverage Δ
unittests	92.59% <100.00%> (+<0.01%)	⬆️

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