afmongodb: Be silent when silently-drop-message is set (ON_ERROR_SILENT)

...otherwise... https://www.youtube.com/watch?v=qfResyFrqlM

[ sorry :-) ]

Signed-off-by: Budai Laszlo <Laszlo.Budai@balabit.com>
libmongo-client: upgraded to 0.1.8