When a neighbor becomes UNREACHABLE which causes neighbor solicitations
to be send only up to every minute. If the medium is very busy this can
easily get lost, basically causing the neighbor never to be reachable
again from the perspective of the sending node. To fix this the backoff
is reduced to its start value, every time a packet is sent to that
neighbor.
While the GNRC_IPV6_NIB_RTR_TIMEOUT is properly handled, it is actually
never fired. Moreover, the router lifetime is set, but never read.
This removes the router lifetime and switches it out for an evtimer
to does the GNRC_IPV6_NIB_RTR_TIMEOUT event.
_is_reachable is only used when GNRC_IPV6_NIB_CONF_ARSM
is enabled, and as such it must be guarded so that clang
doesn't complain about a unused function in case
GNRC_IPV6_NIB_CONF_ARSM is not set
Similar to #7910
Relates to 6473
