Feature: Progressive Messages (#208)

This change is a reworking of the way that messages are sent to clients from the service layer. It is also a reworking of the protocol to ensure that all formulations of query send back events to the client in a deterministic ordering. To support the first change:
* Added a new event that will be sent when a message is generated
* Messages now indicate which Batch (if any) generated them
* Messages now indicate if they were error level
* Removed message storage in Batch objects and BatchSummary objects
* Batch objects no longer have error state

test/Microsoft.SqlTools.ServiceLayer.TestDriver/Tests/TestHelper.cs

@@ -295,7 +295,7 @@ namespace Microsoft.SqlTools.ServiceLayer.TestDriver.Tests
             };
 
             var result = await Driver.SendRequest(QueryExecuteRequest.Type, queryParams);
-            if (result != null && string.IsNullOrEmpty(result.Messages))
+            if (result != null)
             {
                 var eventResult = await Driver.WaitForEvent(QueryExecuteCompleteEvent.Type, timeoutMilliseconds);
                 return eventResult;
@@ -384,6 +384,15 @@ namespace Microsoft.SqlTools.ServiceLayer.TestDriver.Tests
             return result;
         }
 
+        /// <summary>
+        /// Waits for a message to be returned by the service
+        /// </summary>
+        /// <returns>A message from the service layer</returns>
+        public async Task<QueryExecuteMessageParams> WaitForMessage()
+        {
+            return await Driver.WaitForEvent(QueryExecuteMessageEvent.Type);
+        }
+
         public void WriteToFile(string ownerUri, string query)
         {
             lock (fileLock)