Avoid race condition between predicate move and commit (#2392)

- The reason for bug #2338 was that there was a race condition between a mutation and predicate move. Zero was not checking if a predicate is under move before allowing a commit. Thus, a mutation could get proposed in a group, then a move starts, and get committed by Zero (after the move starts).
- This change this issue by ensuring that Zero checks if a predicate is being moved, before allowing commit.
- Any pending transactions are also cancelled once the move starts, so this would only happen as part of a race condition and not afterward.

Mechanism:
- Send the real keys back to Zero, as part of Transaction Context.
- Zero uses these keys to parse the predicate, and checks if that predicate is currently moving. If so, it would abort the transaction.
- Also, check for `_predicate_` being moved. For some reason, if we don't consider this predicate, we could still lose data.
- Before doing a mutation in Dgraph alpha, check if that tablet can be written to.
- Loop until all transactions corresponding to the predicate move are aborted. Only then start the move.

Tangential changes:
- Update the port number for bank integration test.
- Remove the separate key value or clean channel. Make it run as part of the main Node.Run loop.
- Add a max function.
- Small refactoring here and there.