fix a race condition in selecting a unique download path

Created by: herrmannw22

Selecting a unique path was decoupled from the actual file creation, so it was possible for two threads to select the same "unique" path. I fixed this by creating a unique NSFileManager instance, which we lock while performing directory lookups and file creation. Even though this instance is not used when SandCastle is called to detect name collisions, we can still rely on locking it as an abstract resource to ensure correctness of the unique path selection code. Any time we try to read the file list from disk, or modify it (via creation or deletion) we lock the fileManager instance. I created an accessor for it in SDSafariDownload so that SDDownloadManager could use it, but ideally the unique path selection code should be localized to a single location. If we didn't rely on setting download.filename within SDDownloadManager, and instead always do this in SDSafariDownload instances, there would be no need for the accessor.