pgsql: Add BarrierArriveAndDetachExceptLast().

Add BarrierArriveAndDetachExceptLast().

Provide a way for one process to continue the remaining phases of a
(previously) parallel computation alone.  Later patches will use this to
extend Parallel Hash Join.

Author: Melanie Plageman <melanieplageman@gmail.com>
Reviewed-by: Thomas Munro <thomas.munro@gmail.com>
Discussion: https://postgr.es/m/CA%2BhUKG%2BA6ftXPz4oe92%2Bx8Er%2BxpGZqto70-Q_ERwRaSyA%3DafNg%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/7888b0999488511e4266f2134053fa3a6505a155

Modified Files
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src/backend/storage/ipc/barrier.c | 22 ++++++++++++++++++++++
src/include/storage/barrier.h     |  1 +
2 files changed, 23 insertions(+)