* Add on_stall_conditions_changed to EventListener;
Signed-off-by: Tong Zhigao <tongzhigao@pingcap.com>

* Test stall condition changes from normal to other;
Signed-off-by: Tong Zhigao <tongzhigao@pingcap.com>

* Optimize test;
Signed-off-by: Tong Zhigao <tongzhigao@pingcap.com>