riak_ensemble_manager does sync writes

[sargun]

Can we have an option for turning off synchronous writes in the riak_ensemble_manager backend? It's done here:
https://github.com/basho/riak_ensemble/blob/develop/src/riak_ensemble_util.erl#L42

Is there a good reason for this? Some hosting providers / SANs do a full sync on the entire volume, even if one file writes.