Implement build cancelation

[ppalaga]

fixes #127

[ppalaga]

This is a bit different than the original #195 : the cancellation request is not enforced while the DaemonConnector is connecting. It is rather stored in a temporary outbound queue, waiting for the connection and is send to the daemon only after when the connection is established. Could you please review @gnodet ?

[gnodet]

I'd like to keep the ability to cancel the client during the first few seconds while it's trying to connect to a daemon.
I think it currently somewhat works because of #193, so I think the daemon is killed very early when hitting Ctrl+C.

Also, it seems the client is waiting for the daemon to process the cancellation request cleanly. I don't think this is a desirable behaviour. It can slow the response on the client side a few seconds. The client should send the cancel message and exit early.

[gnodet]

I'd like to keep the ability to cancel the client during the first few seconds while it's trying to connect to a daemon.
I think it currently somewhat works because of #193, so I think the daemon is killed very early when hitting Ctrl+C.

Also, it seems the client is waiting for the daemon to process the cancellation request cleanly. I don't think this is a desirable behaviour. It can slow the response on the client side a few seconds. The client should send the cancel message and exit early.

I should have a proposal in a few minutes...

[gnodet]

@ppalaga see gnodet@3a38265

[ppalaga]

Thanks, amended with minor changes in c52dfa3