fixed random assertion error

[rene-ye]

race condition where 2 onRequestComplete() executed and the first one set the value to true before the second one evaluated. place tests inside synchronized to fix.

[codecov-io]

Codecov Report

Merging #597 into dev will increase coverage by 0.01%.
The diff coverage is 33.33%.

@@             Coverage Diff              @@
##                dev     #597      +/-   ##
============================================
+ Coverage     46.54%   46.56%   +0.01%     
- Complexity     2218     2225       +7     
============================================
  Files           108      108              
  Lines         25335    25335              
  Branches       4177     4177              
============================================
+ Hits          11792    11796       +4     
- Misses        11514    11516       +2     
+ Partials       2029     2023       -6

Flag	Coverage Δ	Complexity Δ
#JDBC41	46.37% <33.33%> (+0.09%)	2216 <0> (+7)	⬆️
#JDBC42	46.38% <33.33%> (ø)	2216 <0> (+7)	⬆️

Impacted Files	Coverage Δ	Complexity Δ
...in/java/com/microsoft/sqlserver/jdbc/IOBuffer.java	53.46% <33.33%> (-0.44%)	0 <0> (ø)
...a/com/microsoft/sqlserver/jdbc/PLPInputStream.java	53.01% <0%> (-1.21%)	29% <0%> (-1%)
...c/main/java/com/microsoft/sqlserver/jdbc/Util.java	60.75% <0%> (-0.45%)	87% <0%> (-1%)
...n/java/com/microsoft/sqlserver/jdbc/Parameter.java	62.23% <0%> (-0.22%)	63% <0%> (ø)
...om/microsoft/sqlserver/jdbc/SQLServerBulkCopy.java	52.51% <0%> (-0.07%)	240% <0%> (+1%)
...rc/main/java/com/microsoft/sqlserver/jdbc/dtv.java	63.24% <0%> (+0.24%)	0% <0%> (ø)	⬇️
...ncurrentlinkedhashmap/ConcurrentLinkedHashMap.java	39.82% <0%> (+0.43%)	45% <0%> (+1%)	⬆️
...m/microsoft/sqlserver/jdbc/SQLServerResultSet.java	33.14% <0%> (+0.44%)	245% <0%> (+5%)	⬆️
...rc/main/java/com/microsoft/sqlserver/jdbc/DDC.java	44.92% <0%> (+1.35%)	105% <0%> (+2%)	⬆️

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