21639: Reenable prefetch, but not on every read()

21639: Reenable prefetch, but not on every read()

Only do prefetch every 128 invocations of read().

This should dramatically reduce the overhead of computing prefetch
while still getting some or moste of the benefits of prefetching.

Indeed, benchmarking suggests that this prefetching strategy, by
advising the kernel to map blocks into RAM, may actually improve
throughput on the high end.

Arvados-DCO-1.1-Signed-off-by: Peter Amstutz <peter.amstutz@curii.com>