Fix creating SQLServerDataTable being O(n^2) issue

[peterbae]

Addresses issue #497.

[codecov-io]

Codecov Report

Merging #514 into dev will increase coverage by 0.04%.
The diff coverage is 72.72%.

@@             Coverage Diff              @@
##                dev     #514      +/-   ##
============================================
+ Coverage     46.51%   46.56%   +0.04%     
+ Complexity     2218     2213       -5     
============================================
  Files           108      108              
  Lines         25312    25304       -8     
  Branches       4181     4176       -5     
============================================
+ Hits          11775    11782       +7     
+ Misses        11501    11494       -7     
+ Partials       2036     2028       -8

Flag	Coverage Δ	Complexity Δ
#JDBC41	46.32% <72.72%> (-0.06%)	2202 <3> (-7)
#JDBC42	46.39% <72.72%> (+0.14%)	2209 <3> (+3)	⬆️

Impacted Files	Coverage Δ	Complexity Δ
...c/main/java/com/microsoft/sqlserver/jdbc/Util.java	60.75% <100%> (+1.57%)	87 <2> (-1)	⬇️
...rc/main/java/com/microsoft/sqlserver/jdbc/TVP.java	45.22% <33.33%> (+0.03%)	27 <0> (ø)	⬇️
...m/microsoft/sqlserver/jdbc/SQLServerDataTable.java	58.73% <75%> (+0.66%)	23 <1> (ø)	⬇️
...va/com/microsoft/sqlserver/jdbc/ThreePartName.java	71.42% <0%> (-14.29%)	7% <0%> (-1%)
...om/microsoft/sqlserver/jdbc/ReaderInputStream.java	44.94% <0%> (-2.25%)	16% <0%> (ø)
...m/microsoft/sqlserver/jdbc/SQLServerResultSet.java	32.57% <0%> (-0.69%)	239% <0%> (-6%)
...m/microsoft/sqlserver/jdbc/SQLServerStatement.java	59.47% <0%> (+0.1%)	131% <0%> (+1%)	⬆️
...rc/main/java/com/microsoft/sqlserver/jdbc/dtv.java	63.24% <0%> (+0.12%)	0% <0%> (ø)	⬇️
...om/microsoft/sqlserver/jdbc/SQLServerBulkCopy.java	52.7% <0%> (+0.19%)	239% <0%> (ø)	⬇️
...ncurrentlinkedhashmap/ConcurrentLinkedHashMap.java	42.42% <0%> (+0.21%)	46% <0%> (ø)	⬇️
... and 3 more

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[alexnixon]

LGTM, just a couple of minor comments.

[alexnixon]

I think naming this columNames would be clearer

[alexnixon]

I'd rather an NPE than silently fail to check dupes if someone messes up null handling.

Better IMO would be defining the local variable like Set<String> columnNames = new HashSet<>(); and omitting this check, but I don't know if that'd violate the project's coding style.

[peterbae]

@alexnixon Thanks for the review. I added the same logic for TVP since it was also applicable there, and made minor changes according to your suggestions.

[AfsanehR-zz]

let's also add a test for checking duplicate column