Avoid race condition between predicate move and commit

[manishrjain]

• The reason for bug Inserts can be lost during a predicate move #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.

This change is