Have LocalState store and check the lock ID, and strictly enforce
unlocking with the correct ID.
This isn't required for local lock correctness, as we track the file
descriptor to unlock, but it does provide a varification that locking
and unlocking is done correctly throughout terraform.
Add the LockUnlock methods to LocalState and BackupState.
The implementation for LocalState will be platform specific. We will use
OS-native locking on the state files, speficially locking whichever
state file we intend to write to.
