Introduce the async savepoint management methods in the SqlConnection class

Author: AlexUstinov

Orm/Xtensive.Orm/Sql/SqlConnection.cs

@@ -354,13 +354,35 @@ public virtual void MakeSavepoint(string name)
     }
 
     /// <summary>
-    /// Rollbacks current transaction to the specified savepoint.
+    /// Asynchronously makes the transaction savepoint.
+    /// </summary>
+    /// <param name="name">The name of the savepoint.</param>
+    /// <param name="token">The cancellation token to terminate execution if needed.</param>
+    public virtual Task MakeSavepointAsync(string name, CancellationToken token = default)
+    {
+      MakeSavepoint(name);
+      return Task.CompletedTask;
+    }
+
+    /// <summary>
+    /// Rolls back current transaction to the specified savepoint.
     /// </summary>
     /// <param name="name">The name of the savepoint.</param>
     public virtual void RollbackToSavepoint(string name) => throw SqlHelper.NotSupported(ServerFeatures.Savepoints);
 
     /// <summary>
-    /// Releases the savepoint with the specfied name.
+    /// Asynchronously rolls back current transaction to the specified savepoint.
+    /// </summary>
+    /// <param name="name">The name of the savepoint.</param>
+    /// <param name="token">The cancellation token to terminate execution if needed.</param>
+    public virtual Task RollbackToSavepointAsync(string name, CancellationToken token = default)
+    {
+      RollbackToSavepoint(name);
+      return Task.CompletedTask;
+    }
+
+    /// <summary>
+    /// Releases the savepoint with the specified name.
     /// </summary>
     /// <param name="name">The name of the savepoint.</param>
     public virtual void ReleaseSavepoint(string name)
@@ -370,6 +392,17 @@ public virtual void ReleaseSavepoint(string name)
       // default impl. will fail on rollback
     }
 
+    /// <summary>
+    /// Asynchronously releases the savepoint with the specified name.
+    /// </summary>
+    /// <param name="name">The name of the savepoint.</param>
+    /// <param name="token">The cancellation token to terminate execution if needed.</param>
+    public virtual Task ReleaseSavepointAsync(string name, CancellationToken token = default)
+    {
+      ReleaseSavepoint(name);
+      return Task.CompletedTask;
+    }
+
     /// <inheritdoc/>
     public void Dispose()
     {