Ticket #508 - part 2 - lock-free access to FrontendConfig structure
    
    https://fedorahosted.org/389/ticket/508
    Reviewed by: mreynolds (Thanks!)
    Branch: master
    Fix Description: When computing the idletimeout, only check the config
    anon_limits_dn if the connection c_dn is NULL.
    Platforms tested: RHEL6 x86_64
    Flag Day: no
    Doc impact: no
    (cherry picked from commit 3d212a547210e03deeab8e72456cefda117c1394)