Return cached fedmsg.meta.msg2usernames(...)

def msg2usernames(msg, **config):
    ''' Return cached fedmsg.meta.msg2usernames(...) '''

    if not _cache.is_configured:
        _cache.configure(**config['fmn.rules.cache'])

    key = "|".join(['usernames', msg['msg_id']]).encode('utf-8')
    creator = lambda: fedmsg.meta.msg2usernames(msg, **config)
    return _cache.get_or_create(key, creator)