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)'
```
Before this PR when encountering null values with some columns which is specified as `NOT NULL`, null values will not be filtered,thi behavior does not match with the original load behavior.
Second column alignment logic has bug :
```
template <typename ColumnInserterFn>
void align_variant_by_name_and_type(ColumnObject& dst, const ColumnObject& src, size_t row_cnt,
ColumnInserterFn inserter) {
CHECK(dst.is_finalized() && src.is_finalized());
// Use rows() here instead of size(), since size() will check_consistency
// but we could not check_consistency since num_rows will be upgraded even
// if src and dst is empty, we just increase the num_rows of dst and fill
// num_rows of default values when meet new data
size_t num_rows = dst.rows();
```
