[Routing] reduced recursion in dumper

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | sensiolabs/SensioFrameworkExtraBundle/issues/275
| License       | MIT

This reduces recursion in the route dumper, avoiding issues with xdebug's recursion limit.