refactor nested group processing: add new code
    
    https://fedorahosted.org/sssd/ticket/1784
    
    1. initialization (main-req), returns members of input group
    2. evaluate group members (group)
    3. perform individual search (no-deref) or dereference attribute (deref)
    4a. no-deref
      1. perform a lookup depending on the type of the member object
      2. all direct members are evaluated first
      3. then we step down in nesting level and evaluate nested groups
    4b. deref
      1. perform a dereference lookup on member attribute
      2. all direct members are evaluated first
      3. then we step down in nesting level and evaluate nested groups
    
    Tevent request flow:
    
                      main-req
                          |
                        group
               |------------------------|
            no-deref                  deref
               |                        |
      |----|------|---------|           |
    user group unknown   recurse     recurse
                /   \    | | | ...   | | | ...
              user group group       group