Messages will be consumed when the side channel registers (#3573)

* Messages will be consumed when the side channel registers

* addressing the comment

* Update com.unity.ml-agents/Runtime/Communicator/RpcCommunicator.cs

Co-Authored-By: Chris Elion <chris.elion@unity3d.com>

Co-authored-by: Chris Elion <celion@gmail.com>

com.unity.ml-agents/Runtime/Communicator/RpcCommunicator.cs

                     "A side channel with type index {0} is already registered. You cannot register multiple " +
                     "side channels of the same id.", channelId));
             }
+
+            var numMessages = m_CachedMessages.Count;
+            for (int i = 0; i < numMessages; i++)
+            {
+                var cachedMessage = m_CachedMessages.Dequeue();
+                if (channelId == cachedMessage.ChannelId)
+                {
+                    sideChannel.OnMessageReceived(cachedMessage.Message);
+                }
+                else
+                {
+                    m_CachedMessages.Enqueue(cachedMessage);
+                }
+            }
             m_SideChannels.Add(channelId, sideChannel);
         }