ripngd: retrofit the 'timer basic' command to the new northbound model

ripngd: retrofit the 'timer basic' command to the new northbound model

Trivial conversion. Use the northbound 'apply_finish()' callback
so we'll call ripng_event() only once even if we change the three
RIPng timers at the same time.

Convert the timers to uint16_t to match their representation in
the YANG model.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>