A pause succeeds when its server's snapshot space is full

Saving a snapshot now leaves the room each server keeps for pausing the sandboxes it runs. When that room is short, the server makes room while the pause waits, up to about a minute: it compresses saved memory images, moves kept snapshots to other servers, and copies paused sandboxes to storage and shrinks them. If there is still no room, the sandbox is paused all the same: the server holds its memory and saves it once there is room, and a wake meanwhile resumes it at once. Such a pause used to fail. Only a sandbox too near the end of its time to save its memory later is not paused: the pause fails with no_capacity (409, retryable) and the sandbox keeps running, its memory intact. A snapshot the server has no room for is refused with host_capacity (409); retry it later. See pause, wake, extend.