Adjust fold-objects timeout

* Revert part of 6e7027e9bb6e4fe3838e98ab31d36abd83aa0a77. It is
  preferrable to use the request timeout over gen_server:call timoeuts.
  Further, these timeouts have been removed in the client in the master
  branch.
* Make the fold objects request timeout configurable, using
  `fold_objects_timeout`.
* Update riakc dependency to get features added in
* basho/riak-erlang-client#160
* Update riak_repl_pb_api (to get a consistent dependency on riak_pb)