Implement NullBackend as a /dev/null FileCacher backend.
cmsMake update

It now uses TaskTypes (Fixes #131).
It was also fixed to work with task versions.
Never delete anything from a FileCacher.

The same file could be used by other structures present in the
database.
Fix some details in Luca's patch.

 * Add comments;
 * Use NullBackend for FileCacher.