Update cmsMake to task versioning and to use TaskTypes (fixes #131).
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.
FileCacher: add method to completely destroy the cache.
Destroy the cache after having finished testing.

It would be meaningful to save the cache for later testing, but
that would require some better thought architecture to make it
locatable by later executions of cmsMake.