Expose a way to calculate total storage amount used by the system [JIRA: RCS-339]

[kellymclaughlin]

[cmeiklejohn]

From looking at the requirements of the related basho/riak_cs_control#39 issue, it seems like it also needs a way to estimate a breakdown of usage by user, return the top users, space remaining, and take into account storage space used by items in Riak, metadata, stats, and the like.

[shino]

Just for information.
#723 also requires riak's cluster-wide disk usage information ( "cluster-wide minimum" of each node's free percentage" ). I guess it needs additional feature in riak side.

[shino]

Memo: Detailed storage stats #1120 is partial solution for this. it's not a raw disk usage in total but something that are invisible for users or normal storage stats does not show, such as lingering writing or (pending|scheduled)_delete volume estimate.