Ticket 48038 - logging should be pluggable
    
    Description:  We need to pass in a logging function to nunc-stans,
                  so we can log nunc-stan information in the DS errors
                  log.  There are also "log start" and "log_close"
                  functions that can be passed in as well.
    
                  If a log function is not set in the thrpool config, then
                  the logging will be sent to syslog.
    
    https://fedorahosted.org/389/ticket/48038/
    
    Reviewed by: rmeggins(Thanks!)