1. Print the last few tablets of decommission backend in fe.log for debug.
2. OlapTableSink should get replica on alive Backends, not only available Backends.
3. When decommission multi Backends, we should drop the redundant replicas before creating a new one.
4. Replicas on decommissioning Backends should be not added to catalog again.
5. Decommissioning Backends should not be chosen as destination of tablet repairing.
1. Add Config.max_routine_load_concurrent_task_num instead of the old one
2. Fix a bug that SHOW ALTER TABLE COLUMN may throw Nullpointer exception
3. Fix some misspelling of docs
