fix legacy planner grammer
fix nereids planner parsing
fix cases
forbid auto range partition with null column
fix CreateTableStmt with auto partition and some partition items.
1 and 2 are about #31585
doc pr: apache/doris-website#488
1. fix race condition problem when get tablet load index
2. change tablet search algorithm from random to round-robin for random distribution table when load_to_single_tablet set to false
For better performance and elasticity, we move memtable from loadchannel to
sink, VTabletSinkV2 is introduced, then there are VTabletWriter and
VTabletSinkV2 distributing rows to tablets. where clauses on mvs are
executed in VTabletWriter, while VTabletSinkV2 needs it too. So common code
is moved to row distribution.
Actually, we can layer code by rows' data flow, then the code is much more
understood and maintainable.
ScanNode -> Sink/Writer (RowDistribution -> IndexChannel / DeltaWriter)
