pgsql: Match the buffer usage tracking for leader and worker backends.

Match the buffer usage tracking for leader and worker backends.

In the leader backend, we don't track the buffer usage for ExecutorStart
phase whereas in worker backend we track it for ExecutorStart phase as
well. This leads to different value for buffer usage stats for the
parallel and non-parallel query. Change the code so that worker backend
also starts tracking buffer usage after ExecutorStart.

Author: Amit Kapila and Robert Haas
Reviewed-by: Robert Haas and Andres Freund
Backpatch-through: 9.6 where this code was introduced
Discussion: https://postgr.es/m/86137f17-1dfb-42f9-7421-82fd786b04a1@anayrat.info

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/ccc84a956b00bdd4c162ca8951dd1ff7890cb51f

Modified Files
--------------
src/backend/executor/execParallel.c | 12 +++++++++---
1 file changed, 9 insertions(+), 3 deletions(-)