FCPClientReferenceImplementation: Remove obsolete JavaDoc

- The class used to be abstract, which is why we had this comment.
  Now it is final, so we don't need it anymore.
- Make the function final to stress the above.
FCPClientReferenceImplementation.stop(): Fix wrong logging

- It is not clear here whether it was a timeout or InterruptedException
or some other issue, so don't say it was a timeout.
- Its possible that we didn't even try to fcp_Unsubscribe() if the
connection was null already. So add special logging for that case.
SubscriptionManager: Add code quality TODO

- A Notification is something which is produced by types which implement
EventSource, they just happen to be Persistent here. So we should eat
EventSource, not Persistent.
- Not resolving this immediately since it requires changing the callers
as well and I want to get a new WOT release out ASAP.