Commands keep running while a server prepares to pause a sandbox

When a server paused a busy sandbox on its own (for example, when its disk ran short), it refused every command from the moment it decided to pause, which could be minutes before the sandbox stopped running, and GET /v1/sandboxes/{id} still returned running meanwhile. Commands are now accepted until the sandbox is paused, the pause is reported immediately, and the next command wakes the sandbox. If the server stops running a sandbox without reporting why, exec and file calls now return 503 sandbox_unreachable (retryable, nothing ran) with a hint to restart it with POST /v1/sandboxes/{id}:restart, instead of 409 not_running for a sandbox whose state is running.