support for partitioned input in giraph

When the graph we need to work on is already partitioned into a fixed number of buckets, with properties such as high edge-locality, low fan-out to other buckets, etc. [for instance using techniques such as https://people.cam.cornell.edu/~jugander/papers/wsdm13-blp.pdf ]

we should be able to partition our graph based on such a mapping, to improve local-requests, etc. thus avoiding huge network communication. this diff is especially useful we repeatedly run algorithms on top of the same graph. In such case, we can compute the partitioning once & then use it to speed up processing, (also requiring less network bandwidth, etc.) for the rest applications on same/ similar graphs.

The diff is big & partly reviewed by my colleagues. Putting it up for review