journal: optimize iteration by skipping exhausted files

If from a previous iteration we know we are at the end of a journal
file, don't bother looking into the file again. This is complicated by
the fact that the EOF does not have to be permanent (think of
"journalctl -f"). So we also check if the number of entries in the
journal file changed.

This optimization has a similar effect as "journal: optimize iteration:
skip whole files behind current location" had.

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