Keep sandboxes on separate servers

A create takes spreadFrom, up to 16 ids of your sandboxes or snapshots, and starts on a server none of them is on. Every sandbox and snapshot now shows placement, an opaque tag that is the same when two share a server. A copy of a memory snapshot starts only on its snapshot's server, so keep warm sources apart and spread copies across their snapshots: each server starts copies at its own pace rather than refusing them account_busy_here. See the API.