Fixed deadlock when trying to update store while iterating.

[soundvibe]

Added java 14 build in travis.

[codecov]

Codecov Report

Merging #7 into master will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master       #7      +/-   ##
============================================
+ Coverage     89.12%   89.16%   +0.04%     
  Complexity      712      712              
============================================
  Files            29       29              
  Lines          2226     2225       -1     
  Branches        288      286       -2     
============================================
  Hits           1984     1984              
  Misses          158      158              
+ Partials         84       83       -1     

Impacted Files	Coverage Δ	Complexity Δ
...main/java/com/linkedin/paldb/impl/StoreRWImpl.java	88.31% <100.00%> (+0.41%)	50.00 <1.00> (ø)

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