databendlabs / openraft / 18818196201

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feat: add I/O flush progress tracking API

Add `watch_log_progress()` and `watch_vote_progress()` APIs to track
when Raft I/O operations are durably written to storage.

The implementation provides two independent progress streams:

- Log progress: Tracks all I/O operations (vote saves and log appends).
  Returns `FlushPoint` containing the vote and last log ID.

- Vote progress: Tracks only vote changes (new term or leader).
  Returns the flushed `Vote`.

Both APIs expose a `WatchProgress` handle with:
- `get()`: Get current progress state immediately
- `wait_until_ge()`: Wait asynchronously until progress reaches threshold

Additionally, add `wait_until()` method to `WatchReceiver` trait that
accepts a closure for custom conditions, complementing `wait_until_ge()`.

Example usage:

```rust
// Wait for a specific log entry to be flushed
let mut log_progress = raft.watch_log_progress();
let target = Some(FlushPoint::new(
    Vote::new_committed(2, node_id),
    Some(LogId::new(LeaderId::new(2, node_id), 100))
));
log_progress.wait_until_ge(&target).await?;
// Now log entry 100 is guaranteed durable

// Track leadership changes
let mut vote_progress = raft.watch_vote_progress();
let current = vote_progress.get();  // Get current vote immediately
let target = Some(Vote::new_committed(3, 0));
vote_progress.wait_until_ge(&target).await?;
// Now term 3 is guaranteed persisted
```

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