LDAP User Cleanup - stable7

# Yay, LDAP user cleanup

## How does this stuff work?

1. A background jobs traverses through the mappings table (50 users a time, every 23 minutes, hard coded) and checks whether they are still available in LDAP. If it is not the case, they are marked as "deleted".
2. The LDAP backend now supports "deleting" users. This returns true, when the user name provided represents one of the marked ones.
3. There are two new occ commands that help using this feature:
 1. occ ldap:showRemnants displays a table with all users marked as deleted and details to them
 2. occ user:delete <uid> deletes the given user. If a marked LDAP user is passed hereto, he will be cleaned up.

## What are the restrictions?

The detection (and thus cleanup) will only happen, when those two requirements are fulfilled:
 1. the config.php has an entry "ldapUserCleanupEnabled" set to true
 2. all configured LDAP connections are enabled

The first requirement means that this feature is disabled by default and you actively need to enable it globally. We may change the default in future, when it has been proofed as working really well and does not get in the way.

The second requirement is because this feature cannot be limited to one LDAP configuration. First, because we do not save (only cache) to which server a user belongs. This is because one user theoretically can exists on many servers, while others do not. This is a measure to help against accidental data loss.

## How is the workflow?

1. Use ```./occ ldap:showRemnants``` to see the list of LDAP users marked as deleted:
```
+---------------+----------------+-------------+------------------------------------------------+-------------------------+----------------------------+--------+
| ownCloud name | Display Name   | LDAP UID    | LDAP DN                                        | Last Login              | Dir                        | Sharer |
+---------------+----------------+-------------+------------------------------------------------+-------------------------+----------------------------+--------+
| zombie12839   | May, Arlene    | zombie12839 | uid=zombie12839,ou=zombies,dc=owncloud,dc=bzoc | December  8, 2014 23:03 | /home/openldap/zombie12839 | N      |
| zombie972     | Kennedy, Alice | zombie972   | uid=zombie972,ou=zombies,dc=owncloud,dc=bzoc   | December  8, 2014 23:02 | /home/openldap/zombie972   | N      |
+---------------+----------------+-------------+------------------------------------------------+-------------------------+----------------------------+--------+
```
2. Use ```./occ user:deleted zombie972``` to clean up behind the user. He will vanish from the table also.

What is removed upon cleanup?

* local group assignments (verify) (LDAP groups memberships are dealt with a different background job)
* User preferences (DB table oc_preferences)
* User home folder (local one). Mounts not tested, but it is the same as with local users.
* Corresponding entry in oc_storages
* Apps may decide to clean up they data by listening to delete hooks.

Yes, this command also works on local users resp. any registered backend that implements delete user functionality.

## What is left to do?

* [ ] clean up log messages
* [ ] rename "GarbageCollector" to something more reasonable
* [ ] reevaluate background job interval and batch size
* [ ] rename ldap:showRemnants to be consisten with ldap:show-config etc. maybe with a better name?
Forgot about this. It's against stable7 ... this needs to be polished just in master (as there are all methods of `\OC\Preferences` available in `\OCP\IConfig`