Add release process `--mode=safe`

[bashlund]

Right now sns supports two release methods for pods: hard (default) and soft.

The hard release is focused on a quick release process from the Ops perspective. It will stop the old pod version and run the new version in the same transaction, which likely gives a small glitch in uptime.

The soft process takes longer time as as it has both versions running with overlap before retiring the old version.

We would want a safe method which guarantees that there is no overlap between version. Which is desirable when running processes which should never concurrently access the same file. For example a database.