filebackend: improved "adviseStat" performance

* Use the normal page size instead of limiting it way down to the cache
  size. Track the stat information in the pages and load into into the
  stat cache as entries are accessed. This should also be less prone to
  evictions causing HEAD requests (or memcached hits).
* Also bumped CACHE_CHEAP_SIZE up to 500.
* Fix a few doc bits

Change-Id: I8d44a072e7bcc56c83d8d9c8c9ac9864530bccf8