Let a build agent be paused, have a build killed, and end an ask cancelled as it took it (hq ADR 0219)

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.

cmd/mesh-controller/push.go

@@ -1142,6 +1142,11 @@ func raiseTheBus(ctx context.Context, inv *inventory.Inventory, address string)
 	if err := broker.RaiseSeats(js, inventory.MeshSeats(), holders); err != nil {
 		return err
 	}
+	// And each work queue's cancelled set (novox/hq ADR 0219), so a holder taking an ask can ask
+	// whether it was cancelled the moment it took it.
+	if err := broker.RaiseCancelledSets(js, inventory.MeshSeats()); err != nil {
+		return err
+	}
 	// Every module's state (novox/hq ADR 0201), from the catalogue: a bucket exists from
 	// registration, so a module reading one may watch it before its owner runs anywhere. One that
 	// nothing declares any more is said and kept — what it holds is data.