Add netstandard2.0 target for ServerTelemetryChannel and remove Newtonsoft.Json dependency (#800)

[clguiman]

Add netstandard2.0 target for ServerTelemetryChannel and remove Newtonsoft.Json dependency (#800)

Fix Issue #800 .
Replaced JavaScriptSerializer with DataContractjsonSerializer (exists in netstandard 2.0 & net45)
I had to update the BackendResponse member attributes with IsRequired=true so BackoffLogicManagerTest.IfContentIsUnexpectedJsonNullIsReturned won't fail.

• I ran Unit Tests locally.

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• Design discussion issue #
• Changes in public surface reviewed
• CHANGELOG.md updated with one line description of the fix, and a link to the original issue.
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[cijothomas]

Please add a test project targetting 2.0 mimicking this https://github.com/Microsoft/ApplicationInsights-dotnet/tree/develop/Test/ServerTelemetryChannel.Test/NetCore.Tests

[clguiman]

@cijothomas I've added the 2.0 tests

[TimothyMothra]

@clguimanMSFT Can you please update the changelog?

[clguiman]

@MS-TimothyMothra should I add the changes under 2.7.0-beta1?

[TimothyMothra]

@clguimanMSFT you can call it 2.6.2.
we're going to get this out right away.

[cijothomas]

#803 - so that we know this is disabled for 2.0 target

[pharring]

The condition should be on the "RunCodeAnalysis" property. The other properties (particularly Optimize and DefineConstants) should still be set.

[clguiman]

Fixed it

[cijothomas]

@SergeyKanzhelev / @Dmitry-Matveev - Please have a look at this. We have modified build defs to test this (and manually ran them - all good)

[Dmitry-Matveev]

Should we add conflicting Newtonsoft for the test app to make sure our dependency does not get back with the future commits? (and if it does, the test will fail?)