sd-journal: rate-limit tail timestamp refresh during iteration

sd-journal: rate-limit tail timestamp refresh during iteration

journal_file_read_tail_timestamp() is called unconditionally in
next_beyond_location() for every file on every iteration step,
resulting in O(N x files) volatile mmap reads. For large queries
like 'journalctl -n 1000000' this makes the command unusably slow
(~5 minutes on systems with many journal files).

Rather than suppressing the call entirely (which would make the
inotify path fully load-bearing for cross-boot ordering), rate-limit
it to at most once per second per file. This reduces the overhead
from O(N x files) to O(T x files) where T is the iteration time in
seconds, while still providing periodic refresh as a fallback for
any missed inotify events and keeping cross-boot ordering
reasonably fresh.

Embed a RateLimit struct in JournalFile for this purpose.

Measured improvement on a real system: 5:24 -> 2:39 (-51%) for
'journalctl -n 1000000'.

Fixes: #42521
Regression introduced in: #26355 (34af74946e)

Signed-off-by: dongshengyuan <dongshengyuan@uniontech.com>