Add riak_kv_ensemble_backend:handle_down/4

Have the backend monitor the vnode and vnode proxy. The peer will
receive 'DOWN' messages and call riak_kv_ensemble_backend:handle_down/4
as a result.

Part of a fix for basho/riak_kv#985
Merge pull request #990 from basho/bugfix/monitor-vnode-proxy

Add riak_kv_ensemble_backend:handle_down/4

Reviewed-by: lordnull