Fix backup service to have multiple backup instances (#396)

* Allow multiple backups per backup service instance

* Add test to run multiple backups

* Update backup cancelTask method signature

* Fix to have multiple backup instances and add more tests

* Address PR comments

* Remove double new lines

* Add Azure check for backup operations

test/Microsoft.SqlTools.ServiceLayer.UnitTests/DisasterRecovery/BackupOperationStub.cs

@@ -0,0 +1,62 @@
+//
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+//
+
+using Microsoft.SqlServer.Management.Smo;
+using Microsoft.SqlTools.ServiceLayer.Admin;
+using Microsoft.SqlTools.ServiceLayer.DisasterRecovery;
+using Microsoft.SqlTools.ServiceLayer.DisasterRecovery.Contracts;
+using System.Data.SqlClient;
+using System.Threading;
+
+namespace Microsoft.SqlTools.ServiceLayer.UnitTests.DisasterRecovery
+{
+    /// <summary>
+    /// Stub class that implements IBackupOperation
+    /// </summary>
+    public class BackupOperationStub : IBackupOperation
+    {
+        /// <summary>
+        /// Initialize 
+        /// </summary>
+        /// <param name="dataContainer"></param>
+        /// <param name="sqlConnection"></param>
+        public void Initialize(CDataContainer dataContainer, SqlConnection sqlConnection)
+        {
+        }
+
+        /// <summary>
+        /// Return database metadata for backup
+        /// </summary>
+        /// <param name="databaseName"></param>
+        /// <returns></returns>
+        public BackupConfigInfo GetBackupConfigInfo(string databaseName)
+        {
+            return null;
+        }
+
+        /// <summary>
+        /// Set backup input properties
+        /// </summary>
+        /// <param name="input"></param>
+        public void SetBackupInput(BackupInfo input)
+        {
+        }
+
+        /// <summary>
+        /// Execute backup
+        /// </summary>
+        public void PerformBackup()
+        {
+            Thread.Sleep(500);
+        }
+
+        /// <summary>
+        /// Cancel backup
+        /// </summary>
+        public void CancelBackup()
+        {
+        }
+    }
+}