Slotification of partition tuple conversion

Hi,

In [1] , it was shown that the partition tuples are needlessly formed
and deformed during tuple conversion (do_convert_tuple), when the same
operation can be done using tuple slots. This is because the input
slot might already have a deformed tuple.

Attached is a patch tup_convert.patch that does the conversion
directly using input and output tuple slots. This patch is to be
applied on an essential patch posted by Amit Langote in [1] that
arranges for dedicated per-partition slots rather than changing
tupdesc of a single slot. I have rebased that patch from [1] and
attached it here (
Allocate-dedicated-slots-of-partition-tuple_amitlan_rebased.patch).

In tup_convert.patch, wherever ConvertPartitionTupleSlot() and
do_convert_tuple() are used to convert partition tuples, I have
replaced them by a new function ConvertTupleSlot().

ConvertPartitionTupleSlot() is exclusively for conversion of
partition-to-parent and vice versa, so I got rid of this.
do_convert_tuple() seems to be used for tuples belonging to
non-partition tables as well, so I have left such calls untouched. May
be we can "slotify" these tuple conversions later as a separate task.

[1] https://www.postgresql.org/message-id/e4f9d743-cd4b-efb0-7574-da21d86a7f36%40lab.ntt.co.jp

--
Thanks,
-Amit Khandekar
EnterpriseDB Corporation
The Postgres Database Company