A queued ask could only be waited out and a running build only ended by
stopping the machine, which redelivered it elsewhere. The holder now serves
current, kill, pause and resume on its own machine's subjects; a kill ends
the build's process group and labelled containers and settles the ask as
failed, killed by hand; pause is kept in the workspace across a restart and
said on the bus. The controller writes cancelled ids to a cancelled set the
holder reads on taking an ask, closing the race a delete alone leaves.