This reverts commit 79b2ee20, which proved
to not be very informative; it looks like the "pgstat wait timeout"
warnings in the buildfarm are just a symptom of running on heavily loaded
machines, and there isn't any weird mechanism causing them to appear.

To try to reduce the frequency of buildfarm failures from this effect,
increase PGSTAT_MAX_WAIT_TIME from 5 seconds to 10.

Also, arrange to not send a fresh inquiry message every single time through
the loop, as that seems more likely to cause problems (by swamping the
collector) than fix them.  We'll now send an inquiry the first time through
the delay loop, and every 640 msec thereafter.