pgsql: Add optimizer and executor support for parallel index-only scans

Add optimizer and executor support for parallel index-only scans.

Commit 5262f7a4fc44f651241d2ff1fa688dd664a34874 added similar support
for parallel index scans; this extends that work to index-only scans.
As with parallel index scans, this requires support from the index AM,
so currently parallel index-only scans will only be possible for btree
indexes.

Rafia Sabih, reviewed and tested by Rahila Syed, Tushar Ahuja,
and Amit Kapila

Discussion: http://postgr.es/m/CAOGQiiPEAs4C=TBp0XShxBvnWXuzGL2u++Hm1=qnCpd6_Mf8Fw@mail.gmail.com

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/0414b26bac09379a4cbf1fbd847d1cee2293c5e4

Modified Files
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src/backend/executor/execParallel.c | 12 ++
src/backend/executor/nodeIndexonlyscan.c | 154 ++++++++++++++++++++++----
src/backend/optimizer/path/indxpath.c | 6 +-
src/include/executor/nodeIndexonlyscan.h | 9 ++
src/include/nodes/execnodes.h | 2 +
src/test/regress/expected/select_parallel.out | 29 ++++-
src/test/regress/sql/select_parallel.sql | 5 +
7 files changed, 191 insertions(+), 26 deletions(-)