Implement a random load balancer

[jjwilke]

Run a load balancing phase that just generates a random distribution. Perform all serialization required to migrate objects.

For now, let's move forward with a global map implementation. The algorithm should be:

1. For its local elements, each node randomly decides who to "give" their local data
2. Compute the serialization sizes of each of the elements. Post Isends for the migrating data.
3. Allgather the new locations and sizes of each element.
4. Using the result of the allgather, update the index->rank map and compute new local elements. Post recvs of the correct size for the incoming elements.
5. Unpack the new local elements and clean up.