ES6 Refactor: dialects / abstract (#6046)

* Make abstract ConnectionManager an ES6 class

* ES6 refactor of abstract ConnectionManager

let, const, arrow functions, property shorthands

* ES6 refactor of AbstractDialect

classes, export default

* Make AbstractQuery an ES6 class

* ES6 refactor of AbstractQuery

let, const, arrow functions, for of

* ES6 refactor of abstract QueryGenerator

use let, const, arrow functions, method shorthands, for..of, sometimes
template strings.
Changes variable declarations so they are declared where initialized
(instead of on the top of the function) as let/const is not hoisted anymore.
Removes some IIFEs in favor of block-scoped variables.
Does _not_ yet refactor lodash template strings to ES6 template strings.

lib/dialects/abstract/index.js

 'use strict';
 
-var AbstractDialect = function() {
-
-};
+class AbstractDialect {}
 
 AbstractDialect.prototype.supports = {
   'DEFAULT': true,
@@ -62,3 +60,5 @@ AbstractDialect.prototype.supports = {
 };
 
 module.exports = AbstractDialect;
+module.exports.AbstractDialect = AbstractDialect;
+module.exports.default = AbstractDialect;