Happy Eyeballs Rev 2: The DNS-ening

Instead of simply trying a V6 connection and following it with a plain V4 connection, this method relies on DNS to make some smarter decisions on resolution. This improves performance and reliability for users (and servers) without IPv6 connectivity at all.

- Switch to a DNS-driven strategy for Happy Eyeballs
- Add a Zipper Merge and CancellableDelay utility to the corups
- Remove state from HappyEyeballsCallback as it largely went unused
- Fix a couple small implementation bugs
- Suppress innocent (unhandled) exceptions.

Dalamud/Utility/Util.cs

@@ -556,6 +556,56 @@ public static class Util
         Process.Start(process);
     }
 
+    /// <summary>
+    /// Perform a "zipper merge" (A, 1, B, 2, C, 3) of multiple enumerables, allowing for lists to end early.
+    /// </summary>
+    /// <param name="sources">A set of enumerable sources to combine.</param>
+    /// <typeparam name="TSource">The resulting type of the merged list to return.</typeparam>
+    /// <returns>A new enumerable, consisting of the final merge of all lists.</returns>
+    public static IEnumerable<TSource> ZipperMerge<TSource>(params IEnumerable<TSource>[] sources)
+    {
+        // Borrowed from https://codereview.stackexchange.com/a/263451, thank you!
+        var enumerators = new IEnumerator<TSource>[sources.Length];
+        try
+        {
+            for (var i = 0; i < sources.Length; i++)
+            {
+                enumerators[i] = sources[i].GetEnumerator();
+            }
+
+            var hasNext = new bool[enumerators.Length];
+
+            bool MoveNext()
+            {
+                var anyHasNext = false;
+                for (var i = 0; i < enumerators.Length; i++)
+                {
+                    anyHasNext |= hasNext[i] = enumerators[i].MoveNext();
+                }
+
+                return anyHasNext;
+            }
+
+            while (MoveNext())
+            {
+                for (var i = 0; i < enumerators.Length; i++)
+                {
+                    if (hasNext[i])
+                    {
+                        yield return enumerators[i].Current;
+                    }
+                }
+            }
+        }
+        finally
+        {
+            foreach (var enumerator in enumerators)
+            {
+                enumerator?.Dispose();
+            }
+        }
+    }
+
     /// <summary>
     /// Dispose this object.
     /// </summary>