pgpool: Add volatile qualifier.

Add volatile qualifier.

In Pgpool-II main, local pointer array "my_backend_status" is not
volatile qualified, while it should have been.
Also in initialize_shared_mem_objects() and sync_backend_from_watchdog():

my_backend_status[i] = &(BACKEND_INFO(i).backend_status);

should have been:

my_backend_status[i] = (volatile BACKEND_STATUS *) &(BACKEND_INFO(i).backend_status);

Without these volatile qualifiers, *(my_backend_status[i]) could read
state value because of compiler optimization. Since
my_backend_status[i] is referred to in popular VALID_BACKEND macro, we
should fix it in all supported branches.

Author: Tatsuo Ishii <ishii(at)postgresql(dot)org>
Discussion: https://www.postgresql.org/message-id/20260701.101148.451717746326656928.ishii%40postgresql.org
Backpatch-through: v4.3

Branch
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V4_7_STABLE

Details
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https://git.postgresql.org/gitweb?p=pgpool2.git;a=commitdiff;h=84a079e639725b446815cd5e9e41ed009405e6ee

Modified Files
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src/include/pool.h | 2 +-
src/main/pgpool_main.c | 13 +++++++++----
2 files changed, 10 insertions(+), 5 deletions(-)