The old behavior in this situation was to simply delete the file. Since
we now have a lock on this file we don't want to close or delete it, so
instead truncate the file at offset 0.
Fix a number of related tests
Use a DynamoDB table to coodinate state locking in S3.
We use a simple strategy here, defining a key containing the value of
the bucket/key of the state file as the lock. If the keys exists, the
locks fails.
TODO: decide if locks should automatically be expired, or require manual
intervention.
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.
James Bardin