Promisify modules (#6723)

* WIP promisify

* promisify psql

* ability to skip some keys

* dont promisify client object

* remove async

* clone entire module so it has all properties

* add shim for node 6

* ignore sessionStore as well

* ignore pool on psql

src/database/postgres.js

@@ -123,6 +123,8 @@ postgresModule.init = function (callback) {
 			require('./postgres/list')(wrappedDB, postgresModule);
 			require('./postgres/transaction')(db, dbNamespace, postgresModule);
 
+			postgresModule.async = require('../promisify')(postgresModule, ['client', 'sessionStore', 'pool']);
+
 			callback();
 		});
 	});