too low NAPTIME_PER_CYCLE /too many wakeups in walreceiver.c

Hi,

Currently the walreceiver wakeups NAPTIME_PER_CYCLE=100 miliseconds in idle 
state. This is rather frequent. I don't really see any reason to do so.
A nice fix would be to latchify that with WaitLatchOrSocket + a SetLatch in 
the signal handler for shutdown but that seems to be too invasive at that 
point. So I suggest simply increasing NAPTIME_PER_CYCLE to 500 or 1000ms?

Andres

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