sss_client: Defer thread cancellation until completion of nss/pam operations
    
    The client code is not cancellation-safe, an application which
    has cancelled an NSS operation will experience subtle bugs,
    hence thread cancellation is deferred until completion of client
    operations.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/3156
    
    Reviewed-by: Sumit Bose <sbose@redhat.com>
    Reviewed-by: Florian Weimer <fweimer@redhat.com>