1. Fix a memory leak in `collect_iterator.cpp` (Fix#6700)
2. Add a new BE config `max_segment_num_per_rowset` to limit the num of segment in new rowset.(Fix#6701)
3. Make the error msg of stream load more friendly.
* Add streaming load feature. You can execute 'help stream load;' to see more information.
Changed:
* Loading phase of a certain table can be parallelized, to reduce the load job execution time when multi load jobs to a single table.
* Using RocksDB to save the header info of tablets in Backends, to reduce the IO operations and increate speeding of restarting.
Fixed:
* A lot of bugs fixed.
