1. make ColumnObject exception safe
2. introduce FlushContext and construct schema at memtable flush stage to make segment independent from dynamic schema
3. add more test cases
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();
```
