ipa-extdom-exop: add instance counter and limit
    
    The user and group lookups done by the extdom plugin might need some
    time depending on the state of the service (typically SSSD) handling the
    requests.
    
    To avoid that all worker threads are busy waiting on a connect or a
    reply from SSSD and no other request can be handled this patch adds an
    instance counter and an instance limit for the extdom plugin.
    
    By default the limit will be around 80% of the number of worker threads.
    It can be tuned further with the plugin option ipaExtdomMaxInstances
    which must in set in ipaextdommaxinstances and should have an integer
    value larger than 0 and lesser than the number of worker threads.
    
    If the instance limit is reached the extdom plugin will return LDAP_BUSY
    for every new request until the number of instance is again below the
    limit.
    
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>