PostgreSQL database driver (#5861)

* [test/database/list] Fix test list 4 being used in two different tests

* [database/postgres] PostgreSQL database driver

* [database/postgres] Make transactions work based on continuation scope.

* [database/postgres] Implement nested transactions

* eslint --fix

* Add database changes from earlier this week to the PostgreSQL driver.

* Fix typo

* Fix postgres.incrObjectFieldBy returning undefined instead of null when given NaN

* [database/postgres] Fix sortedSetsCard returning an array of strings.

* Update socket.io postgres adapter

* Fix PostgreSQL erroring when multiple updates are made to the same sorted set entry in a single operation.

Add a test case to catch this error.

* Fix lint errors.

* Only prune sessions on one instance in a cluster to avoid deadlocks.

They're caught and handled by the database server, but they spam the logs.

* Fix arguments.slice.

src/pubsub.js

@@ -33,6 +33,8 @@ function get() {
 		pubsub = require('./database/redis/pubsub');
 	} else if (nconf.get('mongo')) {
 		pubsub = require('./database/mongo/pubsub');
+	} else if (nconf.get('postgres')) {
+		pubsub = require('./database/postgres/pubsub');
 	}
 
 	real = pubsub;