Fix namespaces for code moved to separate class libraries (#263)

* Create ServiceHost a service-specific class

* Renaming hosting namespace

* Rename credentials namespace

* Fix namespaces

src/Microsoft.SqlTools.Hosting/Utility/Validate.cs

@@ -6,7 +6,7 @@
 using System;
 using System.Collections.Generic;
 
-namespace Microsoft.SqlTools.ServiceLayer.Utility
+namespace Microsoft.SqlTools.Utility
 {
     /// <summary>
     /// Provides common validation methods to simplify method