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/database/sorted.js

@@ -42,6 +42,21 @@ describe('Sorted Set methods', function () {
 				done();
 			});
 		});
+
+		it('should gracefully handle adding the same element twice', function (done) {
+			db.sortedSetAdd('sorted2', [1, 2], ['value1', 'value1'], function (err) {
+				assert.equal(err, null);
+				assert.equal(arguments.length, 1);
+
+				db.sortedSetScore('sorted2', 'value1', function (err, score) {
+					assert.equal(err, null);
+					assert.equal(score, 2);
+					assert.equal(arguments.length, 2);
+
+					done();
+				});
+			});
+		});
 	});
 
 	describe('sortedSetsAdd()', function () {