pgsql: Try again to fix accumulation of parallel workerinstrumentation

Try again to fix accumulation of parallel worker instrumentation.

When a Gather or Gather Merge node is started and stopped multiple
times, accumulate instrumentation data only once, at the end, instead
of after each execution, to avoid recording inflated totals.

Commit 778e78ae9fa51e58f41cbdc72b293291d02d8984, the previous attempt
at a fix, instead reset the state after every execution, which worked
for the general instrumentation data but had problems for the additional
instrumentation specific to Sort and Hash nodes.

Report by hubert depesz lubaczewski.  Analysis and fix by Amit Kapila,
following a design proposal from Thomas Munro, with a comment tweak
by me.

Discussion: http://postgr.es/m/20171127175631.GA405@depesz.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/8526bcb2df76d5171b4f4d6dc7a97560a73a5eff

Modified Files
--------------
src/backend/executor/execParallel.c           | 26 +++++-------
src/backend/executor/nodeHash.c               | 13 ------
src/backend/executor/nodeSort.c               | 17 --------
src/include/executor/nodeHash.h               |  1 -
src/include/executor/nodeSort.h               |  1 -
src/test/regress/expected/select_parallel.out | 59 ++++++++++++++++++++++++++-
src/test/regress/sql/select_parallel.sql      | 28 ++++++++++++-
7 files changed, 96 insertions(+), 49 deletions(-)