Back-port fix for accumulation of parallel worker instrumentation.

Back-port fix for 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.

This is a back-port of commit 8526bcb2df76d5171b4f4d6dc7a97560a73a5eff
by Amit Kapila.

Discussion: http://postgr.es/m/20171127175631.GA405@depesz.com
Discussion: http://postgr.es/m/CAA4eK1KT3BYj50qWhK5qBF=LDzQCoUVSFZjcK3mHoJJeWA+fNA@mail.gmail.com