Refactoring of Catalog Statistics Counters

The old statistics counters were based on a whole bunch of code duplication for:
* reading catalog statistics counters from the database
* storing them back into the database
* manifesting their recursive structure
  * exactly the same counters for `self` and `subtree` 
* handling delta counters for catalog updates

This replaces the old structs by more general templates, that are easier to maintain. In order to add new fields to the structures it is only necessary to register them in `cvmfs/catalog_counters.h`. There are no more positions (except their actual usage) where bookkeeping of catalog counters needs to happen.