fix: heartbeat connection lacks the assignment of connectionTimeout

src/main/java/com/actiontech/dble/net/connection/AbstractConnection.java

@@ -338,7 +338,7 @@ public abstract class AbstractConnection implements Connection {
             if (buffer != null) {
                 recycle(buffer);
             }
-            this.cleanup(null);
+            this.cleanup("it will not writeDirectly because connection has been closed.");
             return;
         }
 

src/main/java/com/actiontech/dble/net/connection/BackendConnection.java

@@ -30,6 +30,7 @@ public class BackendConnection extends PooledConnection {
         super(channel, socketWR);
         this.instance = instance;
         DbInstanceConfig config = instance.getConfig();
+        this.connectionTimeout = config.getPoolConfig().getConnectionTimeout();
         this.host = config.getIp();
         this.port = config.getPort();
         this.lastTime = TimeUtil.currentTimeMillis();