crimson/os/seastore/journal/circular_bounded_journal: find the real

crimson/os/seastore/journal/circular_bounded_journal: find the real
journal tail on startup

Recover the correct journal tail during replay by scanning
JOURNAL_TAIL records instead of relying solely on the on-disk
header, which may be stale if a crash occurs between record
write and header update.

The journal header is stored *separately* from the (append-only)
journal and is not updated atomically with journal writes,
so it may lag behind after a crash.
Treat the journal as the source of truth and rebuild the tail from persisted records.

Replay is reorganized into three passes:
1) discover latest journal tail
2) rebuild allocation map
3) replay deltas

Signed-off-by: Xuehan Xu <xuxuehan@qianxin.com>