Bursts of creates from one custom image spread across servers

Many sandboxes created at once from the same custom image are now placed round-robin across the servers that hold the image, and servers are ranked by their measured CPU and memory use, not only by what is booked on them. A server with the image's start template is preferred only when its load is close to the least-loaded server's. The image is also copied ahead to enough servers to take a burst the size of its last one, at most 16 sandboxes per server, so starts stay fast and one server no longer takes most of a burst while others are idle.