riemann: Update to latest LogThrDestDriver

LogThrDestDriver was changed on merge-queue/3.5 again, update to the
latest snapshot API.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>
riemann: Move the plugin glue to a separate file

Signed-off-by: Gergely Nagy <algernon@balabit.hu>