Add index reformatting support to multi-backend

Fixes #572

Applies the changes from:

```
b50d1df73aee20c4260e72d73fdfcf6493603636
3e1ec4348e320754ac7f9db3805ec82aacbe365c
371f48066eab05301b8aea4df12eb408c4aadf48
58170cdf00033d23e553ff9103ee3027f708be45
f2c82db82743860aa05099c506b8e67143f09800
```

as determined by `git log 1.3.0..HEAD -s -p src/riak_kv_multi_backend.erl`
in the `riak_kv` repo, skipping commits that were just changing tests.
Update xref with multi backend changes
Dialyzer fix in multi backend

Note, this change isn't in the kv version of multi backend, mostly
because Riak CS is more strict about dialyzer than riak_kv is at the
moment.
Merge pull request #576 from basho/bugfix/multi-backend-reformat-indexes

Support reformat indexes in multibackend