Previously, we randomly picked a worker to assign a range to.
Now, we pick a worker that has the least number of range granules
already assigned, ultimately distributing the workload in a more
efficient manner.
Future iterations should also consider the number of read/writes
that a worker is already handling when picking a worker to assign
a range to. This could prevent us from assigning a range to a worker
that is already hot.
Josh Slocum