Support reformat indexes in multibackend

Fixes #572

## Background

* [Original Riak issue](basho/riak_kv#499)

* [The main Riak kv fix](basho/riak_kv#504) (the squashed PR is basho/riak_kv#516)

The changes that were made to the multi backend in Riak KV were not ported
to the Riak CS specific multi backend. This PR addresses that.

## The fix

See the first commit (135a10ee2584b4531b452daf92c3d06333abf3c4) message for details about how this patch was applied.

## Testing

1. Convince yourself that the current implementation (develop branch) is broken: Run Riak 1.3.1
and try the `riak-admin reformat-indexes` call. Note that it breaks because
the Riak CS multibackend doesn't support the command.

2. For this test, you'll run this branch of Riak CS the whole time. First,
start a new cluster/node with Riak 1.3.0 or earlier. Write some data into it with Riak
CS. Move the datadir, upgrade the node to 1.3.1 and then copy the data dir back in.
Start the node and now see if you can succesfully run the `riak-admin reformat-indexes`
command.