Riak client abstraction layer for bag support

Instead of using `riakc_pb_socket` directly, add `riak_cs_riak_client` that understands CS specific types like users and buckets. `riak_cs_riak_client` wraps one or more `riakc_pb_socket` processes. All Webmachine resource callbacks and gc/access daemons use them. 

This PR includes also:
- multibag support (this is motivation for introducing `riak_cs_riak_client`)
- bug fix of premature connection release ba13361