Avoid key fragmentation in FileBackend

* Make sure that global backends do not prefix the cache keys
  with local wiki IDs for container and file stat entries.
  This causes fragmentation and breaks invalidation.

Change-Id: Ic02d6e41f828dc82cc07c7d24c8af29c46392a9c
Merge "Avoid key fragmentation in FileBackend" into wmf/1.23wmf15