1. fixed some error regressions (results error with big nstance_num due to incorrect order by).
2. if set parallel_fragment_exec_instance_num to 0, the concurrency in the Pipeline execution engine will automatically be set to half of the number of CPU cores.
3. add limit to parallel_fragment_exec_instance_num that it cannot be set to more than fe.conf::max_instance_num(Default: 128)
```
mysql [(none)]>set parallel_fragment_exec_instance_num = 514;
ERROR 1231 (42000): errCode = 2, detailMessage = Variable 'parallel_fragment_exec_instance_num' can't be set to the value of '514(Should not be set to more than 128)'
```
d20b5f90d8