Switching from ISqlConnection to DbConnection

This is a fairly minor change that will save tons of time as we develop
this service. The DbConnection and associated Db* abstract classes
ask for synchronous versions of the code and allow the addition of async
code. The SqlClient implementation already implements Db* abstract
classes, so we can piggy back off that for our dependency injection layer.

Tests and existing code has been updated to handle the change, as well

src/Microsoft.SqlTools.ServiceLayer/LanguageServices/LanguageService.cs

@@ -5,6 +5,7 @@
 
 using System;
 using System.Collections.Generic;
+using System.Data.Common;
 using System.Threading;
 using System.Threading.Tasks;
 using Microsoft.SqlTools.EditorServices.Utility;
@@ -309,7 +310,7 @@ namespace Microsoft.SqlTools.ServiceLayer.LanguageServices
         /// Callback for when a user connection is done processing 
         /// </summary> 
         /// <param name="sqlConnection"></param> 
-        public async Task OnConnection(ISqlConnection sqlConnection) 
+        public async Task OnConnection(DbConnection sqlConnection) 
         { 
             await AutoCompleteService.Instance.UpdateAutoCompleteCache(sqlConnection); 
             await Task.FromResult(true);