Learn NUD reachable time

Reachable Time is the time that a node assumes a neighbor is reachable after having received a reachability confirmation. Used by the Neighbor Unreachability Detection algorithm (see RFC 4861 Section 7.3). Although this is computed using random elements to avoid network congestion, the base value can be learned through Router Advertisements.

From RFC 4861 Section 6.3.4. Processing Received Router Advertisements:
```
If the received Reachable Time value is non-zero, the host SHOULD set
its BaseReachableTime variable to the received value. If the new
value differs from the previous value, the host SHOULD re-compute a
new random ReachableTime value. ReachableTime is computed as a
uniformly distributed random value between MIN_RANDOM_FACTOR and
MAX_RANDOM_FACTOR times the BaseReachableTime. Using a random
component eliminates the possibility that Neighbor Unreachability
Detection messages will synchronize with each other.
```

This issue tracks the implementation of learning `BaseReachableTime` by receiving Router Advertisements.