Revisit `persist` mounts

[flxo]

There's no update command and persist mounts should be kept across container updates. The problem is that a container update has the sequence:

• uninstall old version
• install new version

The runtime cannot know whether a "new version" will be installed and therefore remove the persist dir. This might be not optional because cached data in this directly can be of value for "new version".

What are the options for quotas on persist mounts?

[flxo]

Thoughts:

Option 1: Extend api::model::Request::Uninstall with a flag that indicates whether to wipe the data or not. The runtime cannot know if there's a follow up install.

Options 2: Unpin uninstall - wipe the data dir if there's no other container with the same name (not version) present in any repository.

[flxo]

Fixed by #662