A snapshot of a running sandbox, in one call, at any size

POST :snapshot and runtime_snapshot (action create) take a running sandbox: they pause it, capture it and wake it, as a fork does, and answer once the capture is over. A sandbox with volumes is still refused. A copy of a disk snapshot takes the vCPUs, memory and disk you ask for, with a disk at least the saved one; a memory snapshot's copy is still the snapshot's size, and asking for another size answers 400. Every sandbox can now pause, fork and be snapshotted, and every stop keeps the sandbox's files. See snapshots and forks.