FCPInterface.handleGetTrusts(): Adapt to plugin-fcp-rewrite

- Return FCPPluginMessage because the caller sendAllTrusts() will
need one
FCPInterface.handleGetScores(): Adapt to plugin-fcp-rewrite

- Return FCPPluginMessage because the caller sendAllTrusts() will
need one
FCPInterface.sendAllIdentities()/*TrustValues()/*ScoreValues(): Adapt to
plugin-fcp-rewrite

- sendSynchronous() needs a FCPPluginMessage and a timeout now instead
of a SimpleFieldSet and a Bucket