Document the appropriate way to disable UDC.
Makes it possible to disable UDC through System property.

This change also makes UDC not fail if an invalid configuration
value is given, instead it interperts it as the default value.
Merge pull request #2872 from thobe/1.9-udc-configuration

1.9 UDC configuration
A tool for dumping the content of a store.

Very useful when debugging.