[Windows,PATCH] Use faster, higher precision timer API

Hi all

Attached is a patch to switch 9.5 over to using the
GetSystemTimeAsFileTime call instead of separate GetSystemTime and
SystemTimeToFileTime calls.

This patch the first step in improving PostgreSQL's support for Windows
high(er) resolution time.

In addition to requiring one less call into the platform libraries, this
change permits capture of timestamps at up to 100ns precision, instead
of the current 1ms limit. Unfortunately due to platform timer resolution
limitations it will in practice only report with 1ms resolution and
0.1ms precision - or sometimes even as much as 15ms resolution. (If you
want to know more, see the README for
https://github.com/2ndQuadrant/pg_sysdatetime).

On Windows 2012 and Windows 8 I'd like to use the new
GetSystemTimePreciseAsFileTime call instead. As this requires some extra
hoop-jumping to safely and efficiently use it without breaking support
for older platforms I suggest that we start with just switching over to
GetSystemTimeAsFileTime, which has been supported since Windows 2000.
Then more precise time capture can be added in a later patch.

--
 Craig Ringer                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services