fixed misbehaving memory barriers for Visual Studio 2013

seems we are affected by the issue described here:
http://lists.schmorp.de/pipermail/libev/2014q1/002318.html
Merge branch 'devel' of https://github.com/triAGENS/ArangoDB into devel
Merge branch 'devel' of https://github.com/triAGENS/ArangoDB into devel