[FileBackend] Stat caching improvements.

[FileBackend] Stat caching improvements.

Callers often tend to end up calling getFileStat(), at least indirectly,
or in various successive function on the same path. This created RTTs
when the file didn't exist since negatives were not cached. This change
does the following:

* Cache definitive negatives (404s) in the process cache.
  Nothing is cached on failure (like network problems).
* Ignore process cache entries after a brief time period
  so long running scripts do not have overly stale entries.

Change-Id: I356bd9f48281e3c7e7a273778b2aca59c521a0c7