Implement GC algorithm for values in LMDB store

[kenwenzel]

Problem description

This is a follow up issue of #3416.

While the LMDB store is feature complete in comparison to memory and native store it still misses the functionality to evict unused values.

Preferred solution

• track subject, predicate, object and context IDs when statements are removed from the triple indexes
• before finally committing a transaction:
  • test tracked IDs against the triple indexes and remove IDs which are still in use
  • add remaining IDs to some kind of "recycle" list within the value store and remove the associated data

Are you interested in contributing a solution yourself?

Yes

Alternatives you've considered

Implement the GC algorithm on a second thread to impact update performance as few as possible.
While this may be the better option it is harder to implement and I would go with a synchronous implementation for the first PoC.

Anything else?

No response