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