Make client channels public.

This allows them to be modified to set timeouts
as requested in https://github.com/grpc/grpc-swift/issues/85

Examples/Echo/Generated/echo.client.pb.swift

@@ -301,7 +301,7 @@ internal class Echo_EchoUpdateCall {
 
 /// Call methods of this class to make API calls.
 internal class Echo_EchoService {
-  private var channel: Channel
+  public var channel: Channel
 
   /// This metadata will be sent with all requests.
   internal var metadata : Metadata

Plugin/Templates/client.pb.swift

@@ -50,7 +50,7 @@ import SwiftProtobuf
 //-{% endfor %}
 /// Call methods of this class to make API calls.
 {{ access }} class {{ .|serviceclass:file,service }} {
-  private var channel: Channel
+  public var channel: Channel
 
   /// This metadata will be sent with all requests.
   {{ access }} var metadata : Metadata