[WFCORE-196] Don't call ManagedServerProxy.disconnected with the Managed...

...Server lock held
I meant that one in particular, although input in general is most welcome. That one in particular because I was concerned about interfering with the ManagementChannelShutdownHandle stuff on the client connection, and increasing the chances of the client seeing connection closed before getting the response.

I don't think it should be a problem, because if the close handler gets called the channel is somehow closed anyway.