8.1 stats issues on Win32

I've been looking at backporting the stats fix committed to head and 8.2
into 8.1, but realised that it's just not going to work. 8.1 still uses
the dual stats processor stuff, which means that the simplification just
is not possible.

The most obvious result is that autovacuum is very likely to fail on 8.1
if your system load is high enough. (all of stats fail of course, but
autovac is a very common user of this)

Should we note this somewhere?

Oh, and if we were "looking for reasons" to deprecate 8.1, this sounds
like a pretty good one for me. I still think we should keep patchin it,
but it is a very good reason to encourage our users to switch to 8.2.

Now, we could try to fix it there, but we've seen a lot of issues since
day one coming from the "inherit socket in two steps", so even if we can
get this one fix, there could be more lurking around in the dual-process
model. I personally don't think it's worth investing the required time
into fixing that on 8.1.

//Magnus