Made TaskHelpers internal members

TaskHelpers were exposed as public members. This would cause problems since there is a TaskHelpers.Sources NuGet package and the consumer of CacheCow package may also have TaskHelpers.Sources NuGet package. Then, this would may cause compile errors. However, you can always separate them through namespace but it makes it a dirty library if you expose internals to consumers face.

This commit should fix the issue but as both CacheCow.Client and CacheCow.Server have `InternalVisiableTo` global attribute applied for CacheCow.Tests assembly, the project wouldn't build as both project have link reference to TaskHelpers. So, I seperated the test projects, too.