Moving a sparta::Counter doesn't clean up the StatisticSet

[ghost]

After moving a sparta::Counter, the StatisticSet isn't cleaned up -- it has two pointers -- one to the old counter (now deleted) and the new one. The TreeNode supports detaching from parent, but the method onDestroyingChild doesn't appear to be implemented in StatisticSet.

[klingaard]

Updated Statistic_test with an example of this and it's broken:

    TreeNode dummy2(&root, "dummy2", "A second dummy node");
    sparta::StatisticSet moved_stats_set(&dummy2);
    sparta::Counter orig_counter(&moved_stats_set, "moved_stat", "A stat to be moved", Counter::COUNT_NORMAL);
    EXPECT_EQUAL(moved_stats_set.getNumCounters(), 1);
    sparta::Counter moved_counter(std::move(orig_counter));
    EXPECT_EQUAL(moved_stats_set.getNumCounters(), 1);  // <<--- ERROR: reports 2