I want(ed) `riak_cs_riak_client` processes to be a wrappers of PB
connection(s) and hiding underlying complexities (as `riakc_pb_socket`
hides TCP socket).

But I don't stick around to change the direction. Just being holder of
PB connection(s) may be good too.

Is it reasonable to choose the (wishful/future) direction and describe
it in the module comment?

1. Hide PB connections inside `riak_cs_riak_client` processes (close to current impl.)
2. Just manage PB connections and give it to other processes (close to your suggestion).
3. Other (please add your idea)

Currently the module has `master_pbc`, `manifest_pbc` and `block_pbc` that
expose PB connections to outside. If we choose 1. above, these should be
removed and replaced by additional external APIs like `list_keys(BucketName)`,
`mapred(BucketName, ...)` etc.