Start migrate the API for our Atomics to look like std::atomic

This patch change the Atomic class CouchbaseAtomic, and change
the API so that:
  get() => load()
  set() => store()
  cas() => compare_exchange_strong()

and move some of methods that used to be members as
template functions.

Change-Id: I48d873fbf04f40d446444232db44f565ee4fed54
Reviewed-on: http://review.couchbase.org/30135
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>