Step1: updateBeIdTaskMaps, remove unavailable BE and add new alive BE
Step2: process timeout tasks, if a task has already been allocated to BE but not finished before DEFAULT_TASK_TIMEOUT, it will be discarded.
At the same time, the partitions belong to old tasks will be allocated to a new task. The new task with a signature will be added in the queue of needSchedulerRoutineLoadTask.
Step3: process all needSchedulerRoutineLoadTasks, allocate task to BE. The task will be executed by BE.