* filer.sync: fix race condition on first checkpoint save
Initialize lastWriteTime to time.Now() instead of zero time to prevent
the first checkpoint save from being triggered immediately when the
first event arrives. This gives async jobs time to complete and update
the watermark before the checkpoint is saved.
Previously, the zero time caused lastWriteTime.Add(3s).Before(now) to
be true on the first event, triggering an immediate checkpoint save
attempt. But since jobs are processed asynchronously, the watermark
was still 0 (initial value), causing the save to be skipped due to
the 'if offsetTsNs == 0 { return nil }' check.
Fixes#7717
* filer.sync: save checkpoint on graceful shutdown
Add graceful shutdown handling to save the final checkpoint when
filer.sync is terminated. Previously, any sync progress within the
last 3-second checkpoint interval would be lost on shutdown.
Changes:
- Add syncState struct to track current processor and offset save info
- Add atomic pointers syncStateA2B and syncStateB2A for both directions
- Register grace.OnInterrupt hook to save checkpoints on shutdown
- Modify doSubscribeFilerMetaChanges to update sync state atomically
This ensures that when filer.sync is restarted, it resumes from the
correct position instead of potentially replaying old events.
Fixes#7717
