add Series getter for hashCode, modify write cache conditions, enhance test

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[codecov]

Codecov Report

Merging #731 (fcf7e6b) into master (08a9405) will decrease coverage by 0.00%.
The diff coverage is 71.42%.

@@             Coverage Diff              @@
##             master     #731      +/-   ##
============================================
- Coverage     54.95%   54.95%   -0.01%     
- Complexity     3104     3105       +1     
============================================
  Files           739      739              
  Lines         20143    20147       +4     
  Branches       1312     1313       +1     
============================================
+ Hits          11069    11071       +2     
- Misses         8607     8608       +1     
- Partials        467      468       +1     

Impacted Files	Coverage Δ	Complexity Δ
...roic/metadata/elasticsearch/MetadataBackendKV.java	81.74% <60.00%> (+0.21%)	50.00 <0.00> (+1.00)
...rc/main/java/com/spotify/heroic/common/Series.java	80.74% <100.00%> (+0.14%)	41.00 <1.00> (+1.00)
...c/test/AbstractMetadataBackendIndexResourceIT.java	94.47% <100.00%> (ø)	29.00 <0.00> (ø)
...com/spotify/heroic/aggregation/simple/MinBucket.kt	44.44% <0.00%> (-11.12%)	4.00% <0.00%> (-1.00%)

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