Creates from a busy snapshot spread across more servers

A snapshot that many sandboxes start from is now copied to up to four more servers within a minute of its first burst of creates, instead of one more server a minute. Copies go first to servers with room that have started the fewest sandboxes from snapshots in the last hour, newly added servers included. Fewer creates from a snapshot return no_capacity ("The servers that keep this snapshot have no room right now") while the region has room.