servers: MRC:

- Reset file to read-write state when removing a readonly policy again.
- Do not allow to change the replication policy from readonly to a read/write one or vice versa.

git-svn-id: https://xtreemfs.googlecode.com/svn/trunk@2761 f8528748-c2c0-11dd-ba5c-d3b7c2b3b534