fold objects for list keys (proof-of-concept)

## _DO NOT MERGE_

This PR is just to track work, and make it easy to share with others.

## TODO

- [x] Basic proof-of-concept
- [x] Feature parity w/ s3/current-list keys (prefix, delimiter, max-keys, etc.)
- [x] Delimiter key-skipping hack
- [x] Eliminate code-duplication between new and old listing
- [x] Switching of some sort based on capabilities for new and old code
- [x] Profiling and observability info
- [ ] 5-node cluster benchmarks
- [x] Check for memory leaks (at least a simple 'run this 1k times' and check for no memory growth)
- [x] Pure EQC tests
- [x] Share some of the old tests
- [x] Make sure riak_test passes
- [x] Add backward compatibility in riak-erlang-client 2i support branch