Use dictionaries is thread-safe manner.
    
    One of the changes in Python 3 was in the way that dictionaries are
    accessed: the dict.iter{keys,items,values} methods were removed, and the
    return type of dict.{keys,items,values} was changed from returning a
    copy of the dictionary data as a list to returning an iterable view of
    the dictionary data that is modified along with the dictionary. one of
    the implications of this is that it is now impossible to create an
    iterable representation of dictionary data that is safe from
    modifications from other threads, because dictionaries (and now the
    views into the dictionaries) cannot be modified during iteration and
    list(x) works using iteration. Way to go, Python.
    
    This is most visible in the threads module: sometimes wait_all() will
    crash with "RuntimeError: dictionary changed size during iteration."
    Fixed that and some other potential multi-threaded dictionary access in
    threads, dnfpayload and xkl_wrapper through the use of more locks, and
    added a bunch of comments where I needed to convince myself that the
    dictionary access is thread safe.