pgsql: Use MinimalTuple for tuple queues.

Use MinimalTuple for tuple queues.

This representation saves 8 bytes per tuple compared to HeapTuple, and
avoids the need to allocate, copy and free on the receiving side.

Gather can emit the returned MinimalTuple directly, but GatherMerge now
needs to make an explicit copy because it buffers multiple tuples at a
time.  That should be no worse than before.

Reviewed-by: Soumyadeep Chakraborty <soumyadeep2007@gmail.com>
Discussion: https://postgr.es/m/CA%2BhUKG%2B8T_ggoUTAE-U%3DA%2BOcPc4%3DB0nPPHcSfffuQhvXXjML6w%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/cdc7169509113018cc389da740e950c587b5751f

Modified Files
--------------
src/backend/executor/nodeGather.c       | 16 ++++++-------
src/backend/executor/nodeGatherMerge.c  | 40 ++++++++++++++++++---------------
src/backend/executor/tqueue.c           | 30 ++++++++++++-------------
src/backend/optimizer/plan/createplan.c |  8 ++++---
src/include/executor/tqueue.h           |  4 ++--
5 files changed, 51 insertions(+), 47 deletions(-)