Updates based on new changes in 1.5

[sorsaffari]

This issue contains the collection of changes that need to be made in Client Node.js in order to keep it in sync with changes introduced in Grakn 1.5.

• creating a transaction: session.transaction().read();and session.transaction().write();
• instantiation of the Grakn client to be consistent with client Java (@marco-scoppetta @vmax please discuss and clarify this change)
• closing a client: client.close();