This worklog implements the following main components:

1. A main memory representation of the "set of cost constants". This contains
   the cost constants read from the cost constant tables stored in the mysql
   database (see WL#7276). For cost constants that do not have entries in the
   cost constant tables, default values defined in the source code will be used.
      
2. A cache for "cost constant sets". This is responsible for creating the set
   of cost constant, giving new sessions access to the latest versions of the
   cost constants, and for re-reading the cost constant tables in the case
   where these have been updated.