Centralize cache management in User Directory Service

To solve performance issues described in SAK-9860 and elsewhere, ID-EID mappings were added to the cache. The new code was added to the DbUserService class; however, existing cache management code was left in the BaseUserDirectoryService class. To simplify cache maintenance, the code should be kept in one place.

Since standard wisdom is that caches are easiest to tune if kept close to the DB, my inclination is to move the existing ID-to-User cache out of BaseUserDirectoryService and into DbUserService. But we'll see how it goes....