* Give defaultClientName a value when it is null
This prevents calling TryGetAzureRoleInstanceIdNoThrow() repeatedly in the null case.
* Upgrade xunit to 2.3.1
* Upgrade xunit to v2.4.0-beta1 to work around https://github.com/xunit/xunit/issues/1601
* Move xUnit to 2.4.0-beta.1.build3958