FCPClientReferenceImplementation.checkSubscriptions(): Adapt to new throws
declaration of fcp_Subscribe() / fcp_Unsubscribe()

- Throw out the IOException to the caller since we cannot do anything
about it here.
- Notice that we especially do NOT have to update the tables which
contain the "is subscribed?" state: That state is updated by two
possible things: 1) The FCP message handler, which will receive
Unsubscribed messages from WOT. That will work even if we do nothing
here. 2) force_disconnect(), which the caller of this function typically
will end up calling on its own because it makes sense: IOException means
the fred side of the connection has told us that the connection is
closed, so force_disconnect() should be used to cleanup.
- No commit to adapt its callers will follow since the only caller
already catches IOException and then does force_disconnect().
FCPClientReferenceImplementation.force_disconnect(): Adapt to new throws
declaration of fcp_Unsubscribe()
FCPClientReferenceImplementation: Add FIXME for adapting traffic dumping
to event-notifications

- The fact that it only logs the SimpleFieldSet but not the
FCPPluginMessage will not cause any compilation errors, so a FIXME is
needed to ensure that I don't forget about this.
- The "{@link SimpleFieldSet}" is already removed from the JavaDoc so I
don't forget about this.