test for bucket and key listing timeouts

- Add tests to validate that bucket list streaming is working.
- Add tests to validate that timeouts are working correctly for
  all variations of list buckets and list keys (stream and non,
  timeouts too-short and long-enough).
- add intercept in the right place to simulate delays for large
  numbers of keys/buckets returned.

This PR relies on the following PRs:
- https://github.com/basho/riak-erlang-client/pull/100
- https://github.com/basho/riak-erlang-http-client/pull/16
- https://github.com/basho/riak_pb/pull/41

This PR tests the following PRs:
- https://github.com/basho/riak_pb/pull/41
- https://github.com/basho/riak_core/pull/302
- https://github.com/basho/riak_kv/pull/527