Mainly fix the following issues:
1. A null pointer exception is raised when a database or table is dropped. The expected behavior is that the routine load job is stopped.
2. Memory leaks. Batch routine load task submissions are no longer performed, and modifications are submitted separately for each task.
3. Unreasonable task timeout.
Routine load tasks should not be queued in the BE thread pool for execution. The task sent to the BE should be executed immediately, otherwise the task in the FE will be timeout first. Eventually leads to constant timeout for all subsequent tasks.
4. All routine load job should be scheduled once it being submitted. Not waiting the available BE slot. Otherwise, all later submitted jobs may not be scheduled forever.
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