Creates from a custom image wait for a copy instead of failing

A sandbox that starts from an image runs only on the servers that keep a copy of it. When those servers are full and another server in the region has room, POST /v1/sandboxes now copies the image to the server with the most room and waits for the copy, then starts the sandbox there. The wait depends on the image's size: about 30 seconds for a 6 GB image, and at most 90 seconds. Previously the create returned 503 no_capacity until the copy was made in the background, which could take several minutes. A create still returns no_capacity immediately when no server in the region has room. The extra copies are kept for a day after the last such create.