Raise a check's store without durability and remove what earlier holders left (hq issue 306)

On the control node the store-bound packages of the controller's suite ran
five to seven times slower than on any other holder, and every controller
check that landed there ran past the suite's thirty minutes: a throwaway
store flushing to a disk the mesh's own store, bus and forge keep busy.
A store that lives for one check needs no crash safety.

A holder recreated mid-check left the check's store and bus running, and
the redelivery went to another machine, so nothing removed them: eleven
pairs across four machines. A starting holder has taken nothing, so every
container labelled with an ask of the seat is an earlier holder's.

cmd/mesh-builder/main.go

@@ -129,6 +129,9 @@ func run() error {
 	if h.Paused() {
 		fmt.Fprintf(os.Stderr, "paused (kept in %s): taking no build until resumed\n", pausedFile(workspace))
 	}
+	// **What an earlier holder here left goes before anything is taken** (novox/hq issue 306): a holder
+	// recreated mid-check left the check's store and bus running, and the ask went to another machine.
+	h.removeLeftBehind()
 	machine := link.MachineOverNATSWith(js, on, seat, link.MachineOptions{Paused: h.Paused})
 	defer machine.Close()