[FileRepo] Lazy load large metadata from the DB.

* Previously, large values that could not fit into memcached would not be cached
  and the master DB would be hit for any file loading (djvu files hit this often).

Change-Id: I3ed8ad4a85d6e3084330b56c3b48ee76103bd2b8
(cherry picked from commit dcbdcf07810580e820e53b24b6c3b9d4c3ca37a3)