Use pthread keys for thread local storage
    
    This interface is slower but also more portable, and more importantly
    it provides a way to specify destructor that is called when a thread
    is canceled so we stop leaking memory.
    
    Signed-off-by: Simo Sorce <simo@redhat.com>
    Reviewed-by: Robbie Harwood <rharwood@redhat.com>
    Merges: #233