Ldap split mapping from Access superclass

This splits off the user and group mapping methods from Access super class, because a) they have nothing to do with accessing LDPA and b) they have nothing to do with any LDAP configuration. This also allows to integrate the clear mappings method and remove them from the static helper class. Furthermore this can be properly tested AND it is necessary for a proper LDAP user clean up feature (well, we could do without, but at some point we need to access the mappings, while we don't need to access LDAP, a configuration, a user manager and other stuff that needs to be initialized at the moment).

cc @MorrisJobke  
Should also be adjusted in `\OC\DB\Connection::insertIfNotExist` then