clusterlin: keep FIFO queue of improvable chunks (preparation)

This introduces a queue of chunks that still need processing, in both
MakeTopological() and OptimizationStep(). This is simultaneously:
* A preparation for introducing randomization, by allowing permuting the
  queue.
* An improvement to the fairness of suboptimal solutions, by distributing
  the work more fairly over chunks.
* An optimization, by avoiding retrying chunks over and over again which
  are already known to be optimal.

src/test/fuzz/cluster_linearize.cpp

@@ -1068,6 +1068,8 @@ FUZZ_TARGET(clusterlin_sfl)
     }
 
     // Loop until optimal.
+    test_fn();
+    sfl.StartOptimizing();
     while (true) {
         test_fn();
         if (!sfl.OptimizeStep()) break;