Update to latest riak_ensemble integrity approach

This is the sibling pull-request for basho/riak_ensemble#37. That pull-request completely changes how `riak_ensemble` performs data validation and integrity checking, completely removing the need for backend based trust/syncing.

This pull-request updates `riak_kv` to work with this new approach. For the most part, that means deleting a bunch of code that is no longer necessary. As well as updating the `ensemble_status` command to no longer reference AAE or trust -- since neither of those concepts apply anymore.

For more details, look at the individual commits.

***Note:*** a discussion with product management needs to happen concerning the new output of `ensemble_status` as well as how we should expose the new integrity checking settings to `riak.conf`. The old discussion about `trust` no longer applies since that setting has been removed. I think the approach as implemented in this pull-request is good enough for 2.0 RC But, we need to have that discussion / change things before 2.0 final.

Maybe just byte the bullet and make this configurable in advanced config?