feat(txn): add hash field in txncontext (#146)

This PR adds the hash field in Request and TxnContext. These changes will be needed by dgraph-io/dgraph#7659 which makes the transactions more robust.

Author: NamanJain8

protos/api.proto

@@ -57,6 +57,7 @@ message Request {
 		RDF = 1;
 	}
 	RespFormat resp_format = 14;
+	string hash = 15;
 }
 
 message Uids {
@@ -128,6 +129,7 @@ message TxnContext {
 	bool aborted = 3;
 	repeated string keys = 4;  // List of keys to be used for conflict detection.
 	repeated string preds = 5; // List of predicates involved in this transaction.
+	string hash = 6;
 }
 
 message Check {}