BROOKLYN-46: persist files with 600 permission

Note this seriously affects performance of persisting files (i.e. persisting entity state). It drops from about 5000 to 3000 writes per second on my machine, with just measuring the `FileBasedObjectStore` performance (i.e. ignoring the memento generation etc).

It won't affect performance of object store persistence at all.
It would be worth checking the return values from these set calls and logging a warning if any are false.