Need a way to wait for a reply "asynchronously"

[Jakub Jermář]

In the error paths such as the one in async_data_write_forward_fast() when we succeed to send msg, but fail to forward callid, it may be desirable to be able to wait for the reply asynchronously. In the context of the above example, asynchronous waiting for the reply would basically mean marking the message with a special flag telling the async framework to discard the message as soon as the reply arrives.

Without this feature, even the error paths need to wait for the reply synchronously (or else the message will leak), which can prevent forward progress of the task at hand until the one which owes the reply is forcibly killed (eg. because it is still waiting for some other message such as callid from the above example).

[Jakub Jermář]

mainline,1463 introduced async_forget() and mainline,1464 used it in async_data_read_forward_fast() and async_data_write_forward_fast().