Stopping a sandbox keeps writes made just before the stop, even when its agent is busy

When a sandbox whose disk is kept (or that has a volume attached read-write) is stopped, Runtime asks the guest to write its pending data to disk before the machine ends. If the guest's agent was briefly unable to take that request, for example while the sandbox was out of memory, the request was not retried, and data written in the last few seconds before the stop could be missing from the disk. The request is now retried until the guest takes it, for up to 12 seconds. The stop itself still returns immediately.