FCPPluginClient: Fix constructor for intra-node connections to take a reference to the client.
- If the client is running within the same node, and thus the FCPConnectionHandler is null, we need to keep a reference to the client's message handler so we can actually talk to it.
- Also improve the JavaDoc of both constructors:
* Clarify what the situation is with networked and non-networked FCP.
* Don't talk about the FCPConnectionHandler in the constructor which doesn't take one. Talking about it makes sense only if you read the JavaDoc of the other constructor before, which is not necessarily always the case.
* Don't refer to networked FCP as "regular" case and non-networked as "special" case. Both are equally valid uses.