Fix tuple routing in cases where tuple descriptors don't match.

The previous coding failed to work correctly when we have a
multi-level partitioned hierarchy where tables at successive levels
have different attribute numbers for the partition key attributes.  To
fix, have each PartitionDispatch object store a standalone
TupleTableSlot initialized with the TupleDesc of the corresponding
partitioned table, along with a TupleConversionMap to map tuples from
the its parent's rowtype to own rowtype.  After tuple routing chooses
a leaf partition, we must use the leaf partition's tuple descriptor,
not the root table's.  To that end, a dedicated TupleTableSlot for
tuple routing is now allocated in EState.

Amit Langote