sd-journal: cache last entry offset and journal file state

sd-journal: cache last entry offset and journal file state

When the offset of the last entry object (or last object for journal
files generated by an old journald) is not changed, the timestamps
should be updated by journal_file_read_tail_timestamp() are unchanged.
So, we can drop to call fstat() in the function.

As, the journal header is always mapped, so we can read the offset and
journal file state without calling fstat.
Still, when the last entry offset is changed, we may need to call fstat()
to read the entry object. But, hopefully the number of fstat() call
can be reduced.