LDAP: Don't reuse a single tevent callback for multiple requests
    
    Several requests (deref, ASQ and SD) were using the same tevent
    callback. This worked fine for quite some time, because the callback
    only used the tevent_req variables. However, a recent patch changed the
    shared sdap_get_generic_done so that it also longer uses the 'state'
    variable.
    
    At that point, all requsts that re-used the sdap_get_generic_done
    request started failing becaus the type of the state variable was
    different.
    
    This patch makes sure the callbacks only manipulate their own data
    types.
    
    Moreover, sdap_get_generic_ext_done() was renamed because it's not
    really a tevent callback.
    
    Reviewed-by: Michal Židek <mzidek@redhat.com>