Forks start on another server when the source's server is full

POST /v1/sandboxes/{id}:fork now returns 503 no_capacity only when no server in the region has room for the copies. When the source's server is full, each copy starts on another server in the region, and the snapshot is copied there first, including large snapshots. The fork's memory limit (a quarter of a server's free memory per fork) now applies to each server the copies can start on, not to the source's server alone.