win: more reliable uv__hrtime precision

Reduces floating-point operations that can cause precision loss.

Thanks to @Arnavion for suggesting the fix:
nodejs/node#1272 (comment)

src/win/util.c

@@ -59,7 +59,7 @@ static char *process_title;
 static CRITICAL_SECTION process_title_lock;
 
 /* Interval (in seconds) of the high-resolution clock. */
-static double hrtime_interval_ = 0;
+static uint64_t hrtime_interval_ = 0;
 
 
 /*
@@ -75,9 +75,9 @@ void uv__util_init() {
    * and precompute its reciprocal. 
    */
   if (QueryPerformanceFrequency(&perf_frequency)) {
-    hrtime_interval_ = 1.0 / perf_frequency.QuadPart;
+    hrtime_interval_ = perf_frequency.QuadPart;
   } else {
-    hrtime_interval_= 0;
+    hrtime_interval_= 1;
   }
 }
 
@@ -484,7 +484,7 @@ uint64_t uv__hrtime(double scale) {
    * performance counter interval, integer math could cause this computation
    * to overflow. Therefore we resort to floating point math.
    */
-  return (uint64_t) ((double) counter.QuadPart * hrtime_interval_ * scale);
+  return (uint64_t) ((counter.QuadPart / hrtime_interval_) * scale);
 }