pgsql: Make archiver process handle barrier events.

Make archiver process handle barrier events.

Commit d75288fb27 made WAL archiver process an auxiliary process.
An auxiliary process needs to handle barrier events but the commit
forgot to make archiver process do that.

Reported-by: Thomas Munro
Author: Fujii Masao
Reviewed-by: Thomas Munro
Discussion: https://postgr.es/m/CA+hUKGLah2w1pWKHonZP_+EQw69=q56AHYwCgEN8GDzsRG_Hgw@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/981524d2e3aa3f28d364c472e34f5386be846811

Modified Files
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src/backend/postmaster/pgarch.c | 41 +++++++++++++++++++++++++++--------------
1 file changed, 27 insertions(+), 14 deletions(-)