Completely hide CouchbaseAtomic for win32

Change-Id: I1a66d40bfb2134a72c88eaaba0cdfdf66ed272dc
Reviewed-on: http://review.couchbase.org/31567
Reviewed-by: Dave Rigby <daver@couchbase.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>