Improve task executor heartbeat handling and cleanup (#12390)

Improve task executor heartbeat handling and cleanup.

### What problem does this PR solve?

- **Reduce lock contention during executor cleanup**: The cleanup lock
is acquired only when removing expired executors, not during regular
heartbeat reporting, reducing potential lock contention.

- **Optimize own heartbeat cleanup**: Each executor removes its own
expired heartbeat using `zremrangebyscore` instead of `zcount` +
`zpopmin`, reducing Redis operations and improving efficiency.

- **Improve cleanup of other executors' heartbeats**: Expired executors
are detected by checking their latest heartbeat, and stale entries are
removed safely.

- **Other improvements**: IP address and PID are captured once at
startup, and unnecessary global declarations are removed.

### Type of change

- [x] Performance Improvement

Co-authored-by: Kevin Hu <kevinhu.sh@gmail.com>

rag/utils/redis_conn.py

@@ -273,6 +273,17 @@ class RedisDB:
             self.__open__()
         return None
 
+    def zremrangebyscore(self, key: str, min: float, max: float):
+        try:
+            res = self.REDIS.zremrangebyscore(key, min, max)
+            return res
+        except Exception as e:
+            logging.warning(
+                f"RedisDB.zremrangebyscore {key} got exception: {e}"
+            )
+            self.__open__()
+        return 0
+
     def incrby(self, key: str, increment: int):
         return self.REDIS.incrby(key, increment)