Reintroduce Limited Windows XP Support

The server codebase work required to reintroduce XP support, assuming that it is only for dev work and has no real performance requirements, includes at least the following:

Provide a template specialization of AtomicWord<LONGLONG> and AtomicWord<ULONGLONG> for Windows XP only. That specialization should use a single global mutex to synchronize all access to AtomicWord<LONGLONG> and AtomicWord<ULONGLONG> instances, rather than using the unavailable atomic intrinsics.

Other non-XP features may also need to be supported.