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/upgrades/1.4.6/delete_sessions.js

@@ -45,10 +45,12 @@ module.exports = {
 					], function (err) {
 						next(err);
 					});
-				} else {
+				} else if (db.client && db.client.collection) {
 					db.client.collection('sessions').deleteMany({}, {}, function (err) {
 						next(err);
 					});
+				} else {
+					next();
 				}
 			},
 		], callback);