Revert the change which adds count values in the state on read

- it is indeed better to delegate the caching to the page cache used in the CountStore
- having less values in the in-memory state will also make the sorting much faster when we need to merge them
  with the one on disk
Expose index selectivity to Cypher

- this commit will ask lucene to provide info about the selectivity of its indexes and it is calculated as follows:
  selectivity = number of different values in the index / total number of entries in the index