title: Does boost asio call async handlers sequentially
body: <p>If I start an asio timer and a receive on a separate thread to my main thread can the handlers ever run at the same time? I presume not and the separate thread calls events sequentially even if they are ready at the same time. To call handlers at the same time asio would have to make its own (3rd) thread or even interrupt one handler to run the other. I can't find documentation that says handlers will be called sequentially even though that seems probable.</p>