Common sense changes

- Don't retry indefinitely for aborting transactions during predicate move. Stop after a dozen tries. TODO: Need to ensure that this doesn't leave the move into a weird state. Needs more testing.
- Swap the order of waiting for server to catch up. First, wait for Zero to send all data until Txn.ReadTs, then get an agreement among replicas to ensure that we're caught up.
- Some more printfs.
- Delete `_dummy_` data key, instead of `dummy`, which has more chances of conflict with user data.
- Clarify some pieces of code.