Add lastModified and createdAt metadata

[malachid]

Consider adding the ability to check the date a particular entry was created or last modified.

[tndhagedorn]

We should also add:

• lastReadAt
• objectSize

All of this metadata can be used by the preload APIs - preload objects only < X bytes, or only read/written within past X days, etc. This would ensure preload continues to increase performance even as the DB grows

Same with cleaning/vacuuming the database over time - rarely read/written objects, or overly large objects, may be prioritized for deletion.

Another option is to add a "can_recycle" (or similar) to the set APIs - to safely flag which data can be cleaned and which cannot.

Anything to prevent unbounded DB growth over time.