filebackend: try to combine SH and EX lock acquisition

* getPathsToLockForOpsInternal() now returns an array in the format LockManager can use
* Also fixed getScopedLocksForOps() for multiwrite backend (it should accept proxy backend paths)
* Updated getScopedFileLocks() docs

Change-Id: Id0dc50c159c5fbc8fca10a9e46c5be23afcb3e9a