Added concurrent HEAD request support for Swift

* This adds a new preloadFileStat() method to FileBackend.
* FileBackend::doOperations() makes use of this instead of primeFileCache().
  The later was mostly useless due to the 'latest' flag, which stat entries
  rarely had (and since they use add(), 'latest' ones do not override the
  non-'latest' ones).
* Simplified primeContainerCache/primeFileCache to no longer accept
  FileOp objects and made sure nothing passes those in anymore.

Change-Id: Iee3878cadd69c3a594c701c0aff81aab5f473de5