Adding basic support for handling vectors in the database loader. (#4138)

* Adding basic support for handling vectors in the database loader.

* Updating the DatabaseLoaderTests to run against an actual database.

* Fixing a variable name to avoid a conflict

* Fixing the DatabaseLoaderTests.IrisSdcaMaximumEntropy test to look in the TestModel folder

* Adding back a type check, returning NaN as the default for float/double, and removing some dead code.

* Fixing up the DatabaseLoader tests to use the Iris TestDatabase

* Remove a call to pipeline.Preview

* Fix the name of loaderColumns1 to loaderColumns

* Fixing up doc comments and removing allocations from the database loader cursor.

* Responding to PR feedback and removing dead code.

* Mark the DatabaseLoaderTests as Windows specific for now.

* Fixing the context writer to not nullref for a null segment

* Ensure the Bindings segments is null if the count is zero.

* Fix tests to build on netfx, and to be skipped on non-Windows.

* Adding a connection timeout to give localdb a chance to initialize

Author: tannergooding

build/Dependencies.props

@@ -45,9 +45,11 @@
   <PropertyGroup>
     <BenchmarkDotNetVersion>0.11.3</BenchmarkDotNetVersion>
     <MicrosoftCodeAnalysisTestingVersion>1.0.0-beta1-63812-02</MicrosoftCodeAnalysisTestingVersion>
+    <MicrosoftMLTestDatabasesPackageVersion>0.0.5-test</MicrosoftMLTestDatabasesPackageVersion>
     <MicrosoftMLTestModelsPackageVersion>0.0.5-test</MicrosoftMLTestModelsPackageVersion>
     <MicrosoftMLTensorFlowTestModelsVersion>0.0.11-test</MicrosoftMLTensorFlowTestModelsVersion>
     <MicrosoftMLOnnxTestModelsVersion>0.0.5-test</MicrosoftMLOnnxTestModelsVersion>
+    <SystemDataSqlClientVersion>4.6.1</SystemDataSqlClientVersion>
   </PropertyGroup>
 
 </Project>