journal: normalize priority of logging sources

The stream event source has a priority of SD_EVENT_PRIORITY_NORMAL+5,
and stdout source +10, but the native and syslog event sources are left
at the default of 0.

As a result, any heavy native or syslog logger can cause starvation of
the other loggers.  This is trivially demonstrated by running:

 dd if=/dev/urandom bs=8k | od | systemd-cat & # native spammer
 systemd-run echo hello & # stream logger
 journalctl --follow --output=verbose --no-pager --identifier=echo &

... and wait, and wait, the "hello" never comes.

Now kill %1, "hello" arrives finally.