Fix casting issue with cache expiration

[jmazanec15]

Issue #, if available:
#203

Description of changes:
Previously, we did not cast KNNSettings.state().getSettingValue(KNNSettings.KNN_CACHE_ITEM_EXPIRY_TIME_MINUTES)) to TimeValue and then get the long value. This was causing the cache to never be rebuilt, causing the circuit breaker limit to exceed 100%. This change fixes that and adds a test to validate it.

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[codecov]

Codecov Report

Merging #215 into master will increase coverage by 0.27%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #215      +/-   ##
============================================
+ Coverage     77.34%   77.61%   +0.27%     
- Complexity      307      308       +1     
============================================
  Files            50       50              
  Lines          1192     1193       +1     
  Branches        101      101              
============================================
+ Hits            922      926       +4     
+ Misses          221      219       -2     
+ Partials         49       48       -1     

Impacted Files	Coverage Δ	Complexity Δ
...istroforelasticsearch/knn/index/KNNIndexCache.java	92.30% <100.00%> (+2.98%)	35.00 <0.00> (+1.00)

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[vamshin]

Good catch. LGTM!