Wicket app optimizations

Here I propose some minor improvements to the Wicket application:

- reuse the same instance of the IResources used as endpoints for JSON and DB use cases
- create/lookup just one instance of IdentifierLoadAccess when reading all World records for the /db use case
- use constants where possible
- set the logging level to ERROR (http://www.techempower.com/benchmarks/#section=code suggests to log as less as possible)