riak_core_node_watcher should tolerate death of riak_core_ring_events

[slfritchie]

1. {links, L1} = process_info(whereis(riak_core_ring_events), links).
2. exit(whereis(riak_core_ring_events), kill).
3. {links, L2} = process_info(whereis(riak_core_ring_events), links).

length(L1) /= length(L2). After step 2, riak_core_node_watcher's link is not in the L2 list.

workaround: exit(whereis(riak_core_node_watcher), kill), which will restart the node watcher, which will in turn re-link to riak_core_ring_events.

[jrwest]

marking as 2.0.1 for the same reasons as #399 (comment)