The partitioning clause is only a very long single line, which is very
hard to interpret for a human. This patch breaks the partitioning
syntax into one line for the partitioning type, and one line per
partition/subpartition.
When partition pruning resulted in an ordered index scan spanning only
one partition, any descending flag for the scan was wrongly discarded,
turning ORDER BY DESC into ORDER BY ASC, and similar problems.
Fixed by correctly passing descending flag in SCAN_TABREQ signal sent
to data nodes.