Add the number of descendant and the number of children to entries

We do have this information in the database, associated with the ParentIdAndRdn, it would be interesting to add them as operational attributes to any entry pulled from the database.

It will require a bit of refactoring of the AbstractBtreePartition.buildEntryDn() method, because we cache the DN to avoid pulling the information everytime.

Another possibility is to get those 2 values only if required.