Save ensemble/cluster state when cluster enabled

Fixes #34 and hopefully basho/riak_kv#983
See #34 for details

Note that even when `riak_ensemble_manager:maybe_save_state/1` fails during
enable, we still want to call `riak_ensemble_manager:state_changed/1` to
ensure that we don't have inconsistent ets tables and running peers.
However, since this activation is not saved we can still send up with
the issue in basho/riak_kv#983.