logging: make stream log formatting simpler, less detailed
    
    All the details like source file, line number or full date are still
    in taskotron.log. But in stream it's more pleasant to read a shorter
    version, taking less space from the left side. You can always use
    --debug option to see everyting even in stream.
    
    Differential Revision: https://phab.qadevel.cloud.fedoraproject.org/D347