Windows NowNanos() overflows

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### Expected behavior
NowNanos() in port/win/env_win.cc is expected to provide an ever increasing clock.

### Actual behavior
This line causes an int64_t overflow: https://github.com/facebook/rocksdb/blame/master/port/win/env_win.cc#L970

### Steps to reproduce the behavior

int main()

{

    // 10000000 is value from QueryPerformanceFrequency()

    unsigned __int64 a = 1413478673602;  // sample QueryPerformanceCounter() value

    a *= (std::nano::den/10000000);

    std::cout << "expected " << a << "\n";

    

    a = 1413478673602;

    a *= (std::nano::den);

    a /= 10000000;

    std::cout << "overflow " << a << "\n";

}

expected 141347867360200
overflow 1152612400007

### suggested fix

uint64_t WinEnvIO::NowNanos() {

  // all std::chrono clocks on windows have the same resolution that is only

  // good enough for microseconds but not nanoseconds

  // On Windows 8 and Windows 2012 Server

  // GetSystemTimePreciseAsFileTime(¤t_time) can be used

  LARGE_INTEGER li;

  QueryPerformanceCounter(&li);

  // Convert to nanoseconds first to avoid loss of precision

  // and divide by frequency

  if (perf_counter_frequency_ < std::nano::den) {

    li.QuadPart *= (std::nano::den / perf_counter_frequency_);

  } else {

    li.QuadPart /= (perf_counter_frequency_ / std::nano::den);

  } // else

  return li.QuadPart;

}