o Minor bugfixes:
    - Switch weighted node selection rule from using a list of doubles
      to using a list of int64_t. This should make the process slightly
      easier to debug and maintain. Needed for fix for bug 6538.

  o Security features:
    - Switch to a completely time-invariant approach for picking nodes
      weighted by bandwidth. Our old approach would run through the
      part of the loop after it had made its choice slightly slower
      than it ran through the part of the loop before it had made its
      choice. Fix for bug 6538.

  o Code simplifications and refactoring:
    - Move the core of our "choose a weighted element at random" logic
      into its own function, and give it unit tests.  Now the logic is
      testable, and a little less fragile too.