Fix executor for network requests

[natario1]

Fixes #646

[codecov]

Codecov Report

Merging #648 into master will decrease coverage by 0.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master     #648      +/-   ##
============================================
- Coverage     52.88%   52.86%   -0.02%     
  Complexity     1676     1676              
============================================
  Files           131      131              
  Lines         10146    10146              
  Branches       1408     1408              
============================================
- Hits           5366     5364       -2     
- Misses         4338     4339       +1     
- Partials        442      443       +1

Impacted Files	Coverage Δ	Complexity Δ
Parse/src/main/java/com/parse/ParseRequest.java	82.95% <100%> (ø)	18 <0> (ø)	⬇️
...se/src/main/java/com/parse/ParseKeyValueCache.java	45% <0%> (-2%)	16% <0%> (ø)

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[Jawnnypoo]

@natario1 Out of curiosity, did you check throughout the rest of the SDK to see if this is happening anywhere else?

[natario1]

@Jawnnypoo it looks the only other ExecutorServices are the one in ParseSQLiteDatabase and one in GCMService... but they are both single threaded so there should be no issue