The old mechanism was to scan the complete plan, searching for a pattern
with a Join, where the outer side included an Append node. The inner
side was duplicated into an InitPlan, with the pg_partition_oid aggregate
to collect the Oids of all the partitions that can match. That was
inefficient and broken: if the duplicated plan was volatile, you might
choose wrong partitions. And scanning the inner side twice can obviously
be slow, if there are a lot of tuples.

Rewrite the way such plans are generated. Instead of using an InitPlan,
inject a PartitionSelector node into the inner side of the join.

Fixes github issues #2100 and #2116.