fix: obsolete wal entires while opening a migrated region (#4993)

* fix: delete obsolete wal entrie while opening a migrated region

* chore: add logs

* chore: rust fmt

* fix: fix fuzz test

src/datanode/src/heartbeat/handler/downgrade_region.rs

@@ -74,6 +74,10 @@ impl HandlerContext {
 
             // Ignores flush request
             if !writable {
+                warn!(
+                    "Region: {region_id} is not writable, flush_timeout: {:?}",
+                    flush_timeout
+                );
                 return self.downgrade_to_follower_gracefully(region_id).await;
             }