Make access control of Channel more strict

Plugin/Templates/client.swift

@@ -23,7 +23,7 @@
 //-{% endfor %}
 /// Call methods of this class to make API calls.
 {{ access }} final class {{ .|serviceclass:file,service }} {
-  public var channel: Channel
+  public private(set) var channel: Channel
 
   /// This metadata will be sent with all requests.
   {{ access }} var metadata : Metadata