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.

test/mocks/databasemock.js

@@ -59,6 +59,14 @@ if (!testDbConfig) {
 		'    "password": "",\n' +
 		'    "database": "nodebb_test"\n' +
 		'}\n' +
+		' or (postgres):\n' +
+		'"test_database": {\n' +
+		'    "host": "127.0.0.1",\n' +
+		'    "port": "5432",\n' +
+		'    "username": "postgres",\n' +
+		'    "password": "",\n' +
+		'    "database": "nodebb_test"\n' +
+		'}\n' +
 		'==========================================================='
 	);
 	winston.error(errorText);