Fix broken tests in pseudo-distributed mode

As it was pointed out, the following tests are currently broken in pseudo-distributed mode:

1) testBspPageRank, testBspPageRankWithAggregatorWriter
These have been broken for a long time AFAIK, and are due to errors in aggregators. Maja's patch (GIRAPH-259) fixes them, so I won't address them here.

2) TestMutateGraphVertex
After GIRAPH-244, Edge doesn't implement Writable anymore. Unfortunately, there was a sneaky place where it was still required: when using Hadoop RPC, BasicRPCCommunications#addEdge() on the server is called through a proxy, and its parameters are expected to be Writable. A simple signature change fixes this.

3) TestGraphPartitioner
I opened GIRAPH-269 on this one. I don't know how we got to the hardcoded number of expected bytes (123), but if that was obtained by running the test itself, then it's not a great test.
For now, I'm simply changing that to 120, which is what we get after the serialization changes in EdgeListVertex, but I'm leaving GIRAPH-269 open since we should replace this with something more robust.
I added a TODO comment so we don't forget it.