Remove remaining timing-dependency in choosing nodes by bandwidth

The old approach, because of its "tmp >= rand_bw &&
!i_has_been_chosen" check, would run through the second part of the
loop slightly slower than the first part.  Now, we remove
i_has_been_chosen, and instead set rand_bw = UINT64_MAX, so that
every instance of the loop will do exactly the same amount of work
regardless of the initial value of rand_bw.

Fix for bug 6538.

changes/bug6538

@@ -2,3 +2,11 @@
     - 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.
+