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/views/install/index.tpl

@@ -102,6 +102,7 @@
 						<select class="form-control" name="database">
 							<option value="redis">Redis</option>
 							<option value="mongo">MongoDB</option>
+							<option value="postgres">PostgreSQL</option>
 						</select>
 					</div>
 					<div class="col-sm-5 help-text" data-help="Leave the fields blank to use the default settings."><!-- IF error -->There was an error connecting to your database. Please try again.<!-- ENDIF error --></div>