Added SimplePager from SonataDoctrinePhpcrAdminBundle

Added SimplePager from SonataDoctrinePhpcrAdminBundle to improve performance of large datasets

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #649, partially #297
| License       | MIT
| SonataDoctrineORMAdminBundle PR        | sonata-project/SonataDoctrineORMAdminBundle#415