Remove dups, add papers on distributed systems and virtualization

There's 2 copies of Amazon Dynamo paper, with layout difference only.

Added COPS and Eiger paper, which discusses how to support causal consistency on large distributed systems (geographical scale).

Added Xen and VM live migration paper, which is the technique used in Xen hypervisor. It is very influential, as Amazon EC2 is built on top of it. I can't find a proper category to put those 2 papers, so I suggest creating an "operating_systems" folder for similar topics.