Remove the file exists check.

[jealous]

Remove multiple files exists check in the shuffle manager. Instead, try the operation and catch the exception if the file does not exist. We hope this could improve the performance on happy paths.

This commit also fixes the warning messages in the build. This closes #16

[codecov[bot]]

Codecov Report

Merging #27 into master will increase coverage by 0.45%. The diff coverage is 48.27%.

@@             Coverage Diff              @@
##             master      #27      +/-   ##
============================================
+ Coverage     75.33%   75.78%   +0.45%     
  Complexity      435      435              
============================================
  Files            30       30              
  Lines          2035     2044       +9     
  Branches        332      332              
============================================
+ Hits           1533     1549      +16     
+ Misses          277      275       -2     
+ Partials        225      220       -5

Impacted Files	Coverage Δ	Complexity Δ
...apache/spark/shuffle/sort/SplashUnsafeSorter.scala	80.79% <ø> (-0.13%)	37 <0> (ø)
.../org/apache/spark/shuffle/SplashObjectWriter.scala	83.9% <100%> (+1.14%)	30 <0> (ø)	:arrow_down:
...k/shuffle/SplashBypassMergeSortShuffleWriter.scala	79.26% <20%> (-1.75%)	20 <0> (ø)
...che/spark/shuffle/SplashShuffleBlockResolver.scala	75.56% <50%> (-1.82%)	34 <1> (-1)
.../main/java/com/memverge/splash/TmpShuffleFile.java	77.27% <55.55%> (+7.7%)	2 <0> (-1)	:arrow_down:
...memverge/splash/shared/SharedFSTmpShuffleFile.java	74.02% <0%> (+2.59%)	15% <0%> (+1%)	:arrow_up:
...om/memverge/splash/shared/SharedFSShuffleFile.java	87.5% <0%> (+12.5%)	9% <0%> (ø)	:arrow_down:
.../apache/spark/shuffle/local/LocalShuffleFile.scala	60% <0%> (+17.14%)	14% <0%> (+1%)	:arrow_up:

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