Implement SyncPoint DB table

- Based on https://github.com/zaneb/heat-convergence-prototype/blob/resumable/converge/sync_point.py
- Stores a record of the dependencies that have been satisfied, and the cached get_resource and get_attr data they supplied
- There's probably a better name, the implementation has changed a lot since I first wrote the code above (update the simulator to match to avoid confusion)
- Updates must be atomic, using "UPDATE ... WHERE ..." form as discussed in http://www.joinfu.com/2015/01/understanding-reservations-concurrency-locking-in-nova/
- Probably use composite primary keys (resource_id & traversal id), so there is a predictable unique key (for fast lookup) but we can still query for e.g. all rows from a single traversal.