Use GetSystemTimeAsFileTime directly in windows gettimeofday

PostgreSQL was calling GetSystemTime followed by SystemTimeToFileTime in the
win32 port gettimeofday function. This is not necessary and limits the reported
precision to the 1ms granularity that the SYSTEMTIME struct can represent. By
using GetSystemTimeAsFileTime we avoid unnecessary conversions and capture
timestamps at 100ns granularity, which is then rounded to 1µs granularity for
storage in a PostgreSQL timestamp.

On most Windows systems this change will actually have no significant effect as
the system timer tick is typically between 1ms and 15ms depending on what timer
resolution currently running applications have requested. You can check this
with clockres.exe from sysinternals. Despite the platform limiation this change
still permits capture of finer timestamps where the system is capable of
producing them and it gets rid of an unnecessary syscall.

Future work may permit use of GetSystemTimePreciseAsFileTime on Windows 8 and
Windows Server 2012 for higher resolution time capture. This call has the same
interface as GetSystemTimeAsFileTime.