[FileRepo] Various code cleanups.

[FileRepo] Various code cleanups.

* Made File::isHashed() wrap FileRepo::getHashLevels(). Removed now-used FileRepo::isHashed().
* Removed FileRepo::simpleClean(). Not useful anymore since the paths in Status errors don't have $IP or upload dirs anymore.
* Removed code in FileRepo::fileExistsBatch() and FileRepo::cleanupBatch() to handle FS file paths, which should never be passed in anymore. Likewise, removed FILES_ONLY parameter.
* Removed FileRepo::append()/appendFinish() stub functions.
* Added FileRepo::assertWritableRepo() function to better handle repos that are read-only by design rather than the hack of overwriting each function (several were missed).
* Added FileBackend::isPathTraversalFree() function and used it in FileRepo::validateFilename() to avoid duplication.
* Tweaked FileRepo::freeTemp() to avoid file locking and made FileRepo::cleanupBatch() return a Status.
* Moved FileRepo::cleanupDeletedBatch() near FileRepo::deleteBatch().
* Added type hinting to a few places.
* Tweaked some misleading doc comments and added function visibility markers.

Change 1:
* Simplified NullRepo to also use assertWritableRepo(). It is currently only used by a single unit test.

Change-Id: I1cd0f4971011772e38e5156f94ffc50325372f28