Renamed some documents in the distributed_systems folder

I renamed the documents:
* beehive.pdf
* megastore.pdf
* pastry.pdf
* rados.pdf
* zookeeper.pdf

To the following names:

* beehive-lookup-performance-for-power-law-query-distributions-in-peer-to-peer-overlays.pdf
* megastore-providing-scalable-highly-available-storage-for-interactive-services.pdf
* pastry-scalable-decentralized-object-location-and-routing-for-large-scale-peer-to-peer-systems.pdf
* rados-a-scalable-reliable-storage-service-for-petabyte-scale-storage-clusters.pdf
* zookeeper-wait-free-coordination-for-internet-scale-systems.pdf