Mark auto-expired key-value pairs in the mutable log as tombstoned

[badrishc]

Feature request type

enhancement

Is your feature request related to a problem? Please describe

When an expiration time is attached to entries in the store, after expiration, they are not removed from the log until (and if) they tier down to storage and get compacted.

We would like to add some way to tombstone these records so that (1) the heap objects are freed in case of the object store; and (2) the log space can be reused for new objects using revivification, in case of the main store.

Describe the solution you'd like

One way to accomplish this is to have a periodic background scanner thread that scans (only the mutable region of) the log, and tombstones any expired entries.

Describe alternatives you've considered

No response

Additional context

See #1097 for a more detailed discussion of this.