Expand partitioned tables in PartDesc order.

Expand partitioned tables in PartDesc order.

Previously, we expanded the inheritance hierarchy in the order in
which find_all_inheritors had locked the tables, but that turns out
to block quite a bit of useful optimization.  For example, a
partition-wise join can't count on two tables with matching bounds
to get expanded in the same order.

Where possible, this change results in expanding partitioned tables in
*bound* order.  Bound order isn't well-defined for a list-partitioned
table with a null-accepting partition or for a list-partitioned table
where the bounds for a single partition are interleaved with other
partitions.  However, when expansion in bound order is possible, it
opens up further opportunities for optimization, such as
strength-reducing MergeAppend to Append when the expansion order
matches the desired sort order.

Patch by me, with cosmetic revisions by Ashutosh Bapat.

Discussion: http://postgr.es/m/CA+TgmoZrKj7kEzcMSum3aXV4eyvvbh9WD=c6m=002WMheDyE3A@mail.gmail.com