[Feature] Maintenance manager

The Maintainence manager collects Maintainer classes which perform maintainence tasks.

Example maintainers:

- Ensure that node order is syncronized to the tree order
- Ensure that content is indexed in the search bundle

A command is provided to execute the maintainers:

````
$ php app/console sulu:content:maintain
````

This command can be run periodically or when upgrading to ensure the consistency of the database(s).