Fix Swarm() hangs

CHANGES.md

@@ -104,6 +104,8 @@ To be released.
     already has and deal with only longer (higher) blocks.
  -  Fixed a bug that occured when `Swarm` was handling multiple responses at the
     same time.
+ -  Fixed a bug that hangs when calling the `Swarm` constructor in certain
+    runtimes. (e.g. Unity3d)
  -  Removed `AddressTransactionSet` which handles handle `Address` to
     `IEnumerable<TxId>` indices, and the following methods in `IStore`:
      -  `IStore.IterateAddresses()`

Libplanet.Tests/Net/SwarmTest.cs

@@ -573,12 +573,26 @@ await Task.WhenAll(
         }
 
         [Fact]
-        public void CanDenyNullParams()
+        public void ThrowArgumentExceptionInConstructor()
         {
             Assert.Throws<ArgumentNullException>(() =>
             {
                 new Swarm(null);
             });
+
+            // Swarm needs host or iceServers.
+            Assert.Throws<ArgumentException>(() =>
+            {
+                new Swarm(new PrivateKey());
+            });
+
+            // Swarm needs host or iceServers.
+            Assert.Throws<ArgumentException>(() =>
+            {
+                new Swarm(
+                    new PrivateKey(),
+                    iceServers: new IceServer[] { });
+            });
         }
 
         [Fact]

Libplanet/Net/Swarm.cs

@@ -46,12 +46,13 @@ public partial class Swarm : ICollection<Peer>, IDisposable
         private readonly AsyncLock _receiveMutex;
         private readonly AsyncLock _blockSyncMutex;
         private readonly string _host;
-        private readonly TurnClient _turnClient;
+        private readonly IList<IceServer> _iceServers;
 
         private readonly ILogger _logger;
 
         private TaskCompletionSource<object> _runningEvent;
         private int? _listenPort;
+        private TurnClient _turnClient;
 
         private NetMQQueue<Message> _replyQueue;
 
@@ -115,15 +116,11 @@ public Swarm(
                 EndPoint = new DnsEndPoint(_host, listenPort.Value);
             }
 
-            if (iceServers != null)
-            {
-                _turnClient = IceServer.CreateTurnClient(iceServers).Result;
-            }
-
-            if (_host == null && _turnClient == null)
+            _iceServers = iceServers?.ToList();
+            if (_host == null && (_iceServers == null || !_iceServers.Any()))
             {
                 throw new ArgumentException(
-                    $"Swarm needs {nameof(host)} or {iceServers}.");
+                    $"Swarm needs {nameof(host)} or {nameof(iceServers)}.");
             }
 
             string loggerId = _privateKey.PublicKey.ToAddress().ToHex();
@@ -399,6 +396,11 @@ public async Task StartAsync<T>(
                 throw new SwarmException("Swarm is already running.");
             }
 
+            if (_iceServers != null)
+            {
+                _turnClient = await IceServer.CreateTurnClient(_iceServers);
+            }
+
             if (_listenPort == null)
             {
                 _listenPort = _router.BindRandomPort("tcp://*");