WL_SOCKET_ACCEPT fairness on Windows

Hi,

Commit 7389aad6 started using WaitEventSetWait() to wait for incoming
connections.  Before that, we used select(), for which we have our own
implementation for Windows.

While hacking on patches to rip a bunch of unused Win32 socket wrapper
code out, I twigged that the old pgwin32_select() code was careful to
report multiple sockets at once by brute force polling of all of them,
while WaitEventSetWait() doesn't do that: it reports just one event,
because that's what the Windows WaitForMultipleEvents() syscall does.
I guess this means you can probably fill up the listen queue of server
socket 1 to prevent server socket 2 from ever being serviced, whereas
on Unix we'll accept one connection at a time from each in round-robin
fashion.

I think we could get the same effect as pgwin32_select() more cheaply
by doing the initial WaitForMultipleEvents() call with the caller's
timeout exactly as we do today, and then, while we have space,
repeatedly calling
WaitForMultipleEvents(handles=&events[last_signaled_event_index + 1],
timeout=0) until it reports timeout.  Windows always reports the
signaled event with the lowest index in the array, so the idea is to
poll the remaining part of the array without waiting, to check for any
more.  In the most common case of FEBE socket waits etc there would be
no extra system call (because it uses nevents=1, so no space for
more), and in the postmaster's main loop there would commonly be only
one extra system call to determine that no other events have been
signaled.

The attached patch shows the idea.  It's using an ugly goto, but I
guess it should be made decent with a real loop; I just didn't want to
change the indentation level for this POC.

I mention this now because I'm not sure whether to consider this an
'open item' for 16, or merely an enhancement for 17.  I guess the
former, because someone might call that a new denial of service
vector.  On the other hand, if you fill up the listen queue for socket
1 with enough vigour, you're also denying service to socket 1, so I
don't know if it's worth worrying about.  Opinions on that?

I don't have Windows to test any of this.  Although this patch passes
on CI, that means nothing, as I don't expect the new code to be
reached, so I'm hoping to find a someone who would be able to set up
such a test on Windows, ie putting elog in to the new code path and
trying to reach it by connecting to two different ports fast enough to
exercise the multiple event case.

Or maybe latch.c really needs its own test suite.