Add extra data to the Stack table for convergence

- See https://github.com/zaneb/heat-convergence-prototype/blob/resumable/converge/stack.py
- Extra fields in the simulator (above) are: prev_tmpl_key, current_traversal, current_deps
- Updates must be atomic w.r.t. current_traversal, using "UPDATE ... WHERE ..." form as discussed in http://www.joinfu.com/2015/01/understanding-reservations-concurrency-locking-in-nova/