fix(unet-channels): Invoking .Clear() on two Dictionaries within the Shutdown-function (#199)

* fix(unet-channels): Invoking .Clear() on two Dictionaries within the Shutdown-function

GetConfig() will try to insert new kvp-pairs even though they already exists. Clearing them fixes this and allow, for example, doing StartHost() -> StopHost() -> StartHost() within the same session.

* changed the fields back to read-only

Author: BabakSamimi

MLAPI/Data/Transports/UNET/UnetTransport.cs

@@ -202,6 +202,8 @@ public override ulong GetCurrentRtt(ulong clientId)
 
         public override void Shutdown()
         {
+            channelIdToName.Clear();
+            channelNameToId.Clear();
             NetworkTransport.Shutdown();
         }