Add handle_down/4 cb for backend behavior

Handle 'DOWN' messages in the peer and callback into the backend
handle_down/4 function.

Part of a fix for basho/riak_kv#985
fix 'DOWN' message handling in peer
Merge pull request #33 from basho/bugfix/monitor-vnode-proxy

Add handle_down/4 cb for backend behavior

Reviewed-by: lordnull