A sandbox's lease counts from when it first runs

A paid sandbox that waits in starting for its server to have room now has its lease renewed while it waits, so it is no longer stopped with time_limit before it ever runs, and its server no longer refuses the start as invalid_lease once the wait used up its lease. No compute is charged for the time spent waiting. diskKept on a stopped sandbox is now true only when POST /v1/sandboxes/{id}:restart starts it from its disk: a stopped sandbox whose server was lost now returns diskKept: false.