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