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

...Server lock held
The AbstractChannelOpenListenerService adds a closeListener which triggers the cancellation of active requests already, so adding another closeListener doing the same should not be required.
Basically the ManagementChannelShutdownHandle should take care of rejecting new requests, however allowing active ones to complete. Where basically the services wait for completion on shutdown, before closing the channel.
In general if we receive a channel close notification we can cancel right away, there is no need to wait for any timeout since the channel won't be usable any longer.