Fix | Fix possible server connection leak if an exception occurs in pooling layer

[cheenamalhotra]

This PR is additional fix for consequences of error Internal .NET Framework Data Provider error 6. that is thrown from DbConnectionPool.CreateObject method when something goes wrong in Pooling layer.

Continued investigations over original issue that was fixed by #637 led to figuring out that the catch blocks in this method simply set newObj to null, leaking resources and TDS connections on server, that can only be garbage collected on client when server drops them.

This causes expensive connection leaks on server, and issue exists in System.Data.SqlClient and Microsoft.Data.SqlClient (<2.0.1).
In Microsoft.Data.SqlClient we fixed this issue with PR #638 and #637, this PR is also a fix that ensures resources are cleaned properly in any similar future events.

[saurabh500]

This change can be tested using the TestTdeServer and injecting transient failures
e.g.

SqlClient/src/Microsoft.Data.SqlClient/tests/FunctionalTests/SqlConnectionBasicTests.cs

Line 18 in 9e3f0d6

using (TestTdsServer server = TestTdsServer.StartTestServer())

Another example of the usage of server is at

SqlClient/src/Microsoft.Data.SqlClient/tests/ManualTests/TracingTests/DiagnosticTest.cs

Line 701 in 6a9e4f1

using (var server = TestTdsServer.StartServerWithQueryEngine(new DiagnosticsQueryEngine(), enableLog: enableServerLogging, methodName: methodName))

[JRahnama]

LGTM.

[cheenamalhotra]

/azp run CI-BuildNPack

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[cheenamalhotra]

This change can be tested using the TestTdeServer and injecting transient failures

Hi @saurabh500 I've added the test case and I can also reproduce Internal .NET Framework Data Provider error 6. with connection leak with the new test "TransientFaultTest". You'd need to revert changes from this PR and #637 (just comment out changes) to reproduce it locally.

In order to capture the leaked connection, you'll need to add breakpoint in exception handling logic in the test and capture heap snapshot. SqlConnection instance no longer contains reference of this "Open" internal connection so it's reference cannot be captured via reflection and hence state is always Closed.

Navigating through Heap Snashot you will come across 1 instance of TdsParserStateObjectNative which has TdsParser reference and it's State is OpenLoggedIn. Since only internal connection reference is dropped in the code where I fixed in this PR, the TCP connection to server is intact and is leaked.