A snapshot of a running sandbox answers sooner

sandbox.snapshot() in every SDK now asks the server once, and the server pauses the sandbox for the capture and wakes it before answering, where the SDKs paused, captured and woke it in three calls of their own. A repeat snapshot of a 4 GiB sandbox took 1.19 s at the median instead of 1.96 s in Virginia. Snapshots and forks also see they are ready sooner on the server.