PIM-1891 : Array hydration for product grid

The purpose of this PR is reduce the time needed to build / load the product grid on large set of data (1.3k attributes, 30k products, 6M values, particularity 200 attributes / product).

After investigation, the doctrine object hydration is the main problem here, so, we introduce an array hydration and optimize the differents queries (count results, get entity ids, fetch data) to retrieve only useful data.

We also introduce a new grid extension, the selector extension, that allows to define which selector must be used for each grid columns (static one defined in datagrid.yml or dynamic one added for an attribute), each selector is able to add a query part.

![flash-control](https://f.cloud.github.com/assets/2104359/2177455/94449434-961a-11e3-8e9e-b8a9b022ec84.jpg)

TODO,
 - [x] join only used values (filter by attribute ids)
 - [x] hydrate only displayed product data as array
 - [x] optimize the ConfigureFlexibleGridListener::getFlexibleAttributes to load only useful, and hydrate as a configuration array used by configurator (volumn, sorter, filter)
 - [x] properly retrieve curent locale / scope
 - [x] get only localized, scoped values, options, metrics, prices
 - [x] add support for any backend type
 - [x] update the formaters to display array values in cells
 - [x] pass data locale when user clicks on a product grid row
 - [x] introduce a SelectorExtension with Selector objects that are able to add some select and join in datasource QB
 - [x] configure selectors in grid_attributes.yml
 - [x] fix translations of grid headers
 - [x] refactor attribute repositories configuration methods
 - [x] inject attributes configuration in the grid configuration
 - [x] fix mass actions + behat
 - [x] fix quick export + behat
 - [x] fix mass delete + behat
 - [x] fix variant group edit + behat
 - [x] add missing specs

Controls,
Bug fix: [no]
Feature addition: [no]
Backwards compatibility break: [yes]
Unit test passes: [yes]
Specs passes: [yes]
Behat scenario passes: [yes]
Checkstyle issues: [no]
ChangeLog updated: [yes]
Fixes the following jira:
 - PIM-1891