pgsql: Remove memory leak protection from Gather and Gather Mergenodes

Remove memory leak protection from Gather and Gather Merge nodes.

Before commit 6b65a7fe62e129d5c2b85cd74d6a91d8f7564608, tqueue.c could
perform tuple remapping and thus leak memory, which is why commit
af33039317ddc4a0e38a02e2255c2bf453115fd2 made TupleQueueReaderNext
run in a short-lived context.  Now, however, tqueue.c has been reduced
to a shadow of its former self, and there shouldn't be any chance of
leaks any more.  Accordingly, remove some tuple copying and memory
context manipulation to speed up processing.

Patch by me, reviewed by Amit Kapila.  Some testing by Rafia Sabih.

Discussion: http://postgr.es/m/CAA4eK1LSDydwrNjmYSNkfJ3ZivGSWH9SVswh6QpNzsMdj_oOQA@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/9f4992e2a9939a4c3d560c2ac58067861ee0029a

Modified Files
--------------
src/backend/executor/nodeGather.c      | 14 ++------------
src/backend/executor/nodeGatherMerge.c | 10 +---------
src/backend/executor/tqueue.c          |  2 ++
3 files changed, 5 insertions(+), 21 deletions(-)