ReadOnly Setting

Added a READONLY settings for constance.

This is helpful in the case where for example a cluster of webservers are behind a PostgreSQL, Redis or other readonly backend slaves.

The setting avoids trying to save a setting if it does not exist in the (slave) backend.

The pull request is not clean, but since there a other changes to my fork, I doubt a clean pull request will be possible from my repository. However the changeset for the READONLY settings should apply cleanly.