Proposal of API change

[Sergio Villar Senin <svillar igalia com>]

Hi,

I suggest changing the API of the send queue replacing the current
synchronous cancel API by its an async version The problem of the
current one is that if you're adding a message using the async call then
calling tny_send_queue_cancel will hang the application because the
outbox folder will be locked by two different threads.

Br