ProviderTests updated to suit async code base.
BugFix: Azure provider plugin install and Caching

The Azure provider plugin did not override the base class install
method. Code changed to correct this.

The StandardMetadataCache used HttpContext.Current. This has been
changed to HttpRuntime to access the cache.
ProviderTests: add missing package xunit.execution
Implement Uninstall method for AzureReader2 and S3Reader2. Fix StackOverflowException in S3Reader2.Install. Stop masking errors within BlobProviderBase.Uninstall's call to Config.Plugins.remove_plugin, as they should not occur.