PIM-2127 : introduce a product pager extension (storage agnostic)

Conflicts:
	src/Pim/Bundle/DataGridBundle/Resources/config/extensions.yml
PIM-2127 : add dummy ODM pager
Merge branch 'master' into mongo/grid-pager
PIM-2136 : add MongoDb pager
PIM-2136 : add product pager
PIM-2136 : add specific pager parameter for orm / mongo
PIM-2136 : use product pager extension for product datasource and orm
pager extension for others grids
PIM-2136 : rename hydrator folders to be consistent with OroGridBundle
PIM-2136 : CS Fix
PIM-2136 : remove useless pager
PIM-2136 : update phpdoc and add exception in methods to implement
PIM-2136 : fix Mongo Pager constructor
Merge pull request #826 from akeneo/mongo/grid-pager

Grid pager (MongoDB support)