PluginReplySender: Add getConnectionIdentifier()

- This allow identifying to which client connection a PluginReplySender belongs.
- It is need for WOT SubscriptionManager (= event-notifications) so it can determine whether a certain client is already subscribed to certain events. Also, it will have to store the connection ID in the database so when events happen in the future it can obtain the PluginReplySender with the matching ID to send the events back to.
- For pure FCP network connections, the data needed for this was already stored: A random per-network-connection identifier at FCPConnectionHandler.connectionIdentifier. It was only a matter of exposing it through a getter.
- For intra-node connections between plugins, a new random ID is stored for each PluginTalker
- The result of getConnectionIdentifier() is not only the connection ID but also contains the client-specified "Identifier" from FCP which was already existing in the code. This is to not break clients which try to maintain multiple connections by using a different "Identifier" field.
The implementation is difficult to understand so I added a lot of text :|

I meanwhile decided that I will rewrite the whole PluginTalker/PluginReplySender stuff as new classes and deprecate the old ones. The existing architecture is doubtable and undocumented, which causes people to use it in very different ways, which makes it difficult to change.
So I see two options:
- We ditch this pull request. I think that would be a bad idea, its better to have more documentation for the old classes once I send a pull request for the new ones, then people can compare them.
- You merge it as is and ignore the minor glitches. This saves me the work of having to improve upon the pull request given that the code will be deprecated soon anyway.