pgsql: Convert SISeg->maxMsgNum to an atomic variable.

Convert SISeg->maxMsgNum to an atomic variable.

Currently, this variable is a uint32 protected by a spinlock. The
spinlock exists only to provide memory barriers, so by converting
the variable to an atomic and using the barrier-providing accessors
in the spinlock's place, we can remove the spinlock.

Author: Yura Sokolov <y(dot)sokolov(at)postgrespro(dot)ru>
Reviewed-by: Heikki Linnakangas <hlinnaka(at)iki(dot)fi>
Reviewed-by: Peter Eisentraut <peter(at)eisentraut(dot)org>
Reviewed-by: Andres Freund <andres(at)anarazel(dot)de>
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/30aa0030-f694-44ef-a19d-6ef7ddb69374%40postgrespro.ru
Discussion: https://postgr.es/m/alAJeRRzehDjLaF1%40nathan

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/df3978c2340d79c6319f8239a69abbd27b1069df

Modified Files
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src/backend/storage/ipc/sinvaladt.c | 54 ++++++++++++++-----------------------
1 file changed, 20 insertions(+), 34 deletions(-)