Re-create a routing table entry if the next hop is changed.

The next-hop address did not get updated in the routing table
in case an entry for the destination already existed.

This patch resolves the issue by removing the entry and
having it re-created from scratch.

The issue causes a routing error triggering reconstruction of
the DODAG through version increase.

In case of somewhat frequent downward traffic in not (yet) stabilized DODAG
a vicious circle is formed: unstable topology means churn, downward
routing under churn causes reconstruction of DODAG. In this situation
the network does not have chance to stabilize.

We encountered a constant churn caused by this bug
in a network of 50 nodes and a periodic traffic (a packet every 5
seconds) generated at the root.

More info and a PCAP demonstrating the issue can be found here:
https://github.com/contiki-os/contiki/issues/496