pgsql: Clean up handling of constraint_exclusion and enable_partition_p

Clean up handling of constraint_exclusion and enable_partition_pruning. The interaction of these parameters was a bit confused/confusing, and in fact v11 entirely misses the opportunity to apply partition constraints when a partition is accessed directly (rather than indirectly from its parent).

pgsql: Clean up handling of constraint_exclusion and enable_partition_p

Clean up handling of constraint_exclusion and enable_partition_pruning. The interaction of these parameters was a bit confused/confusing, and in fact v11 entirely misses the opportunity to apply partition constraints when a partition is accessed directly (rather than indirectly from its parent).