add gsets support

[russelldb]

This is the same as branch rdb/bet365/gsets which was already merged to basho/riak_pb develop after full review and testing. The difference here is that it is pointed at develop-2.2, and it has been rebased to bring it down to one commit

https://github.com/basho/riak_kv/pull/1665 https://github.com/basho/riak_test/pull/1302

[bryanhuntesl]

@martincox - one for merging.