pgsql: Allow parallel create index to accumulate buffer usage stats.

Allow parallel create index to accumulate buffer usage stats.

Currently, we don't account for buffer usage incurred by parallel workers
for parallel create index.  This commit allows each worker to record the
buffer usage stats and leader backend to accumulate that stats at the
end of the operation.  This will allow pg_stat_statements to display
correct buffer usage stats for (parallel) create index command.

Reported-by: Julien Rouhaud
Author: Sawada Masahiko
Reviewed-by: Dilip Kumar, Julien Rouhaud and Amit Kapila
Backpatch-through: 11, where this was introduced
Discussion: https://postgr.es/m/20200328151721.GB12854@nol

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/13823ad3b012cfaf986ee4dfbd60190ca07187f1

Modified Files
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src/backend/access/nbtree/nbtsort.c | 39 +++++++++++++++++++++++++++++++++++++
1 file changed, 39 insertions(+)