Revisit time functions

Addressing https://github.com/MSOpenTech/redis/issues/188

- Made gettimeofday use GetSystemTimePreciseAsFileTime when available
- Introduced definitions for gettimeofday_fast and gettimeofday_precise,
  so that calls can be specialized based on specific usage requirements
- Introduced GetHighResRelativeTime, for high-resolution relative time
  measurements on all Windows versions.