[FileBackend] Added doQuickOperations() function for things like purging thumbnails.

[FileBackend] Added doQuickOperations() function for things like purging thumbnails.

* doQuickOperations() lets us do things like purge thumbnails as fast as possible.
  Stat calls, SHA1 checks, and RTTs in general are avoided. It also avoids the
  slowness of lazy population of thumbnail SHA1s in Swift (for those made by rewrite.py).
* Removed supportedOperations() to avoid the extra complexity.
* Made a few variable type and exception documentation cleanups.
* Cleaned up unit test file removal a bit and made some functions private.

Change-Id: I6922368c6af7752a6927d96402519132203108a1