pgsql: Convert Sharedsort's worker counters to atomic variables.

Convert Sharedsort's worker counters to atomic variables.

Currently, currentWorker and workersFinished are ints protected by
a spinlock. By converting them to atomic variables, we can remove
the spinlock. Note that the spinlock also covered the write of
each worker's tape metadata. Removing it is still okay: each
worker writes only its own slot, and the atomic increment that
follows is a full barrier. Furthermore, the leader doesn't read
the slot values until all workers have finished.

Reviewed-by: Zsolt Parragi <zsolt(dot)parragi(at)percona(dot)com>
Tested-by: solai v <solai(dot)cdac(at)gmail(dot)com>
Discussion: https://postgr.es/m/alAJeRRzehDjLaF1%40nathan

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/ee97eef695e8be9153595131f8b63e4b1b3e8196

Modified Files
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src/backend/utils/sort/tuplesort.c | 30 ++++++++----------------------
1 file changed, 8 insertions(+), 22 deletions(-)