Fix RETURNING to work correctly with partition tuple routing.

Fix RETURNING to work correctly with partition tuple routing.

In ExecInsert(), do not switch back to the root partitioned table
ResultRelInfo until after we finish ExecProcessReturning(), so that
RETURNING projection is done using the partition's descriptor.  For
the projection to work correctly, we must initialize the same for each
leaf partition during ModifyTableState initialization.

Amit Langote