Avoid portability issues in autoprewarm.c.

Avoid portability issues in autoprewarm.c.

autoprewarm.c mostly considered the number of blocks it might be dealing
with as being int64.  This is unnecessary, because NBuffers is declared
as int, and there's been no suggestion that we might widen it in the
foreseeable future.  Moreover, using int64 is problematic because the
code expected INT64_FORMAT to work with fscanf(), something we don't
guarantee, and which indeed fails on some older buildfarm members.

On top of that, the module randomly used uint32 rather than int64 variables
to hold block counters in several places, so it would fail anyway if we
ever did have NBuffers wider than that; and it also supposed that pg_qsort
could sort an int64 number of elements, which is wrong on 32-bit machines
(though no doubt a 32-bit machine couldn't actually have that many
buffers).

Hence, change all these variables to plain int.

In passing, avoid shadowing one variable named i with another,
and avoid casting away const in apw_compare_blockinfo.

Discussion: https://postgr.es/m/7773.1525288909@sss.pgh.pa.us