WebOfTrust: Use new plugin FCP interface
FredPluginFCPMessageHandler.ServerSideMessageHandler

- Adaption of class FCPInterface will follow
FCPInterface: Begin implementation of new interface
FredPluginFCPMessageHandler.ServerSideFCPMessageHandler

First steps here are to:
- Rename the message handler function
- Make sure it does not process reply messages because those should not
arrive here: WOT only does send messages to the client which shall
receive a reply using *synchronous* sends. Synchronous sends will have
their reply returned to the blocking send function instead of having it
passed to the message handler. Anything else sent by WOT should not
receive a reply. Thus, reply messages should not arrive in the
async message handling function.

The rest of the body of the function will be adapted in the following
commits.
FCPInterface.handlePluginFCPMessage(): Finish adapting to new interface,
fix bogus data sending

- The designated way of sending a reply FCPPluginMessage with the new
API is to return it, not to send it using the FCPPluginClient. So we do
this now.
(- The FCPPluginClient may still be used for out of band messages, i.e.
non-replies, this is why we pass it to the handleSubscribe)
- The new implementation of errorMessageFCP will follow, as well has
handleSubscribe