pgsql: Poll postmaster less frequently in recovery.

Poll postmaster less frequently in recovery.

Since commits 9f095299 and f98b8476 we don't poll the postmaster
pipe at all during crash recovery on Linux and FreeBSD, but on other
operating systems we were still doing it for every WAL record. Do it
less frequently on operating systems where system calls are required, at
the cost of delaying exit a bit after postmaster death. This avoids
expensive system calls reported to slow down CPU-bound recovery by as
much as 10-30%.

Reviewed-by: Heikki Linnakangas <hlinnaka(at)iki(dot)fi>
Reviewed-by: Fujii Masao <masao(dot)fujii(at)oss(dot)nttdata(dot)com>
Reviewed-by: Michael Paquier <michael(at)paquier(dot)xyz>
Discussion: https://postgr.es/m/CA%2BhUKGK1607VmtrDUHQXrsooU%3Dap4g4R2yaoByWOOA3m8xevUQ%40mail.gmail.com
Discussion: https://postgr.es/m/7261eb39-0369-f2f4-1bb5-62f3b6083b5e@iki.fi

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/57dcc2ef3320de3e1aef6f668601e6ad2bdf88c4

Modified Files
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src/backend/postmaster/startup.c | 25 +++++++++++++++++++++++--
1 file changed, 23 insertions(+), 2 deletions(-)