do not call dp callbacks when responder is shutting down
    
    https://fedorahosted.org/sssd/ticket/1514
    
    We were experiencing crash duting responder shut down. This happened
    when there were some unresolved dp request during the shut down.
    
    The memory hierarchy is main_ctx->specific_ctx->rctx, where
    specific_ctx may be one of the pam, nss, sudo, etc. contexts.
    
    If we try to call dp request callback as a result of responder
    termination, the specific context is already semi freed, which may
    cause crash.