ELAPI: Adding concept of targets
    
    The targets are the destinations which
    caller wants to send the events to.
    The sinks are now on the second level
    under targets and constitute a so called
    fail over chain for a target.
    Such approach eliminates the need for complex
    routing function.
    The dispatcher keeps the list of targets in a collection.
    The element in the collection is the target context.
    Also gispatcher keeps the list of the sinks
    in a separate collection.
    Each target context has a list of the sinks
    associated with this target. But those are just
    pointers (at least for now) to the sinks
    form the list kept by dispatcher.
    
    I had to add some internal debug callbacks
    to be able to see that all the internals of
    the dispatcher are actually in order.
    See the conttent of config file for more comments.
    Also see information posted on SSSD wiki.
    https://fedorahosted.org/sssd/wiki/WikiPage/ELAPIInterface