Require <atomic> or <cstdatomic>

We have more and more code depending on it

Change-Id: I838167fcabf31bcfc439da70350c6f9721e026a1
Reviewed-on: http://review.couchbase.org/41298
Reviewed-by: Dave Rigby <daver@couchbase.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>