Make DisposeAsync fully async in failure case

[bgrainger]

From #872 (comment):

The following call stack is possible:

MySqlConnection.DisposeAsync
MySqlConnection.CloseAsync
ServerSession.ReturnToPool
ConnectionPool.Return
ServerSession.DisposeAsync(IOBehavior.Synchronous, CancellationToken.None).GetAwaiter().GetResult();

The two non-Async methods eventually cause sync-over-async in the failure path.