nghttpx: Randomize backend address backend robin order per thread (2nd Ed.)

This is the 2nd attempt to randomize backend address round robin
order.

The initial attempt of doing this was
fdcdb21c38, which has been reverted via
470f06e5f6 because it broke the session
affinity.

This time, we shuffles seq fields and avoid reordering addrs vector.

src/shrpx_worker.cc

@@ -360,6 +360,10 @@ void Worker::replace_downstream_config(
         addr.seq = seq++;
       }
 
+      util::shuffle(std::begin(shared_addr->addrs),
+                    std::end(shared_addr->addrs), randgen_,
+                    [](auto i, auto j) { std::swap((*i).seq, (*j).seq); });
+
       if (shared_addr->affinity.type == SessionAffinity::NONE) {
         std::map<StringRef, WeightGroup *> wgs;
         size_t num_wgs = 0;

src/util.h

@@ -879,6 +879,26 @@ void random_bytes(OutputIt first, OutputIt last, Generator &gen) {
   std::generate(first, last, [&dis, &gen]() { return dis(gen); });
 }
 
+// Shuffles the range [|first|, |last|] by calling swap function |fun|
+// for each pair.  |fun| takes 2 RandomIt iterators.  If |fun| is
+// noop, no modification is made.
+template <typename RandomIt, typename Generator, typename SwapFun>
+void shuffle(RandomIt first, RandomIt last, Generator &&gen, SwapFun fun) {
+  auto len = std::distance(first, last);
+  if (len < 2) {
+    return;
+  }
+
+  for (unsigned int i = 0; i < static_cast<unsigned int>(len - 1); ++i) {
+    auto dis = std::uniform_int_distribution<unsigned int>(i, len - 1);
+    auto j = dis(gen);
+    if (i == j) {
+      continue;
+    }
+    fun(first + i, first + j);
+  }
+}
+
 template <typename OutputIterator, typename CharT, size_t N>
 OutputIterator copy_lit(OutputIterator it, CharT (&s)[N]) {
   return std::copy_n(s, N - 1, it);