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
only issue client ops if ensembles enabled
fix 'DOWN' message handling in peer
Merge pull request #31 from basho/optimization/only-issue-cmd-if-enabled

only issue client ops if ensembles enabled

Reviewed-by: engelsanchez
Merge pull request #33 from basho/bugfix/monitor-vnode-proxy

Add handle_down/4 cb for backend behavior

Reviewed-by: lordnull
Add ready to start backend callback

This will be used, for example, to indicate that Riak KV ensemble peers
should wait for the riak_kv service to be up before starting to avoid
some nasty races.