Automatically delete clones after N hours of inactivity

Goal

When a clone is inactive during N hours (should be configurable; default: 2), it has to be destroyed unless it is marked as "protected from deletion".

"Inactive" here means:

• no new connections,
• no SQL requests.

"during N hours" here means "at least N hours". It may be several minutes more. N is "global" for Database Lab instance.

TODO / How to implement

❓ implementation details are not defined, first we need design proposal and discussion.

On a separate note, do we need to tell the user somehow (in Database Lab API responses?) what is the current value of N?

Acceptance criteria

As a user, I know that if I don't use a clone, it will be deleted after N hours. I also know that if I mark it "protected" it will never get deleted.