Feature/zl/add interface for pb bucket key active preflist

[zeeshanlakhani]

Note: Bors will fail on this until a new version is referenced in rebar.config incorporating the update in basho/riak_pb#105 and the riak_core update is reviewed/merged.

Description

Part of RIAK-1481.

The active preflist will return primary/fallback partitions and nodes for the available nodes at the time of query. Primary/fallback will be annotated.
This involves updates to our RiakKV WB code, RiakPB, RiakCore, RiakAPI, Riak-Erlang-Http-Client, Riak-Erlang-Client.

*The impetus for adding this to the API started with a mailing list question answered by Charlie Voiselle, http://lists.basho.com/pipermail/riak-users_lists.basho.com/2015-January/016527.html, which involved a snippet of code that we've given clients multiple times. This was then discussed with Russell Brown on HipChat, https://basho.hipchat.com/history/room/867200/2015/01/13?q=enterprising&t=rid-867200#12:23:22.

Other PRs in the series:

• active preflist on bucket key riak_core#705
• add encode, message, and proto for RpbGetBucketKeyPreflistReq|Rest riak_pb#105
• add wm module for GETing (and encoding) active preflist riak_kv#1077
• add get_preflist to http-client riak-erlang-http-client#50
• add get_preflist to erlang-client riak-erlang-client#204

[zeeshanlakhani]

Closing in order to open a new PR against develop first (then we can merge into 2.0).

[zeeshanlakhani]

#75