Add preds field

Moved from GitHub pydgraph/34

Posted by danielmai:

The preds field was added in Dgraph v1.0.8. It tracks the predicates mutated in a transaction and is handled by the client similar to the keys field by merging in the new list with the existing one per transaction.

Corresponding Go client change: Add a Preds field in TxnContext to allow Zero to abort transactions d… · dgraph-io/[email protected] · GitHub

martinmr commented :

Fixed by #41