pgsql: Fix parallel index and index-only scans to fall back to serial.

Fix parallel index and index-only scans to fall back to serial.

Parallel executor nodes can't assume that parallel execution will
happen in every case where the plan calls for it, because it might
not work out that way.  However, parallel index scan and parallel
index-only scan failed to do the right thing here.  Repair.

Amit Kapila, per a report from me.

Discussion: http://postgr.es/m/CAA4eK1Kq5qb_u2AOoda5XBB91vVWz90w=LgtRLgsssriS8pVTw@mail.gmail.com

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/09529a70bb5a77935d086d651c63396767d240d7

Modified Files
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src/backend/executor/nodeIndexonlyscan.c | 72 +++++++++++++------------
src/backend/executor/nodeIndexscan.c     | 90 ++++++++++++++++++++------------
2 files changed, 96 insertions(+), 66 deletions(-)