unix: ensure done_cb gets called after uv_cancel()

Wake up the event loop with uv_async_send() when a request is cancelled.
Ensures the done_cb is run on the next tick of the event loop.

unix: ensure done_cb gets called after uv_cancel()

Wake up the event loop with uv_async_send() when a request is cancelled.
Ensures the done_cb is run on the next tick of the event loop.

Not sending a wakeup signal results in the done_cb not getting called until
another request completes, which may be a long time coming when it's the only
request in the queue or when other requests are executing long-running jobs.

Fixes #669.

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unix: rework uv_cancel() api

Bert Belder informs me the current approach where a request is immediately
cancelled, is impossible to implement on Windows.

Rework the API to alway

unix: rework uv_cancel() api

Bert Belder informs me the current approach where a request is immediately
cancelled, is impossible to implement on Windows.

Rework the API to always invoke the "done" callback with an UV_ECANCELED error
code.

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unix: add uv_cancel()