Ensure nodes don't transition to exiting too soon

Add ready_to_exit/0 callback to riak_kv_vnode and
riak_kv_ensembles:local_ensembles/0 to ensure that nodes don't
transition from leaving to exiting while they still have ensemble peers
in use.
update dialyzer.ignore-warnings for riak_kv_vnode
Merge pull request #926 from basho/ajs/ensemble_remove_node

Ensure nodes don't transition to exiting too soon

Reviewed-by: andrewjstone