Support transactions

[ospaarmann]

Support transactions and create integration tests.

References:

• Integration tests for the Go client: https://github.com/dgraph-io/dgo/tree/master/contrib/integration.
• Specially, testtxn, acctupsert and bank would be good to have.
• Docs at https://docs.dgraph.io/clients/#raw-http
• And the code for any of the clients, Go/Javascript/Java.

ToDo

• [ ] General transaction setup
• [ ] texttxn
• [ ] acctupsert
• [ ] bank
• [ ] Integration tests