Changing runsc checkpoint compression default

[Ayush Ranjan]

Hi everyone,

We’re switching the default for `runsc checkpoint --compression` from `flate-best-speed` to `none` to improve by-default user experience. Some of our testing showed that `flate-best-speed` compression provided negligible size benefits. The uncompressed snapshot format is much faster to checkpoint and restore. It allows for kernel and memory restore to proceed in parallel. It also allow for the optimizations mentioned in https://gvisor.dev/docs/user_guide/checkpoint_restore/#optimizations.

This will result in multiple files within the --image-path directory, so please ensure you’re migrating that entire path rather than individual files. If you still require compression, you can explicitly set --compression=flate-best-speed.