Improve object listing performance and efficiency

When using Riak CS with Riak 1.4 or greater, we can take advantage of paginated
object listing. This allows us to no longer have to do list-keys on the entire
bucket and do a multi-get of all of the objects. This ends up being faster and
cause less load on the cluster. For backward compatibility, this is implemented
as a new FSM, `riak_cs_list_objects_fsm_v2`, which is only used if explicitly
set in the application. The default is false. This commit adds the option to
the `app.config`, so that it's easy for users to see the new flag. It can also
be set in the console like this:

`application:set_env(riak_cs, fold_objects_for_list_keys, true).`

Furthermore, the built in statistics for the new FSM can be seen by turning on
debug logging. This can be done just for the module like this:

`lager:trace_console([{module, riak_cs_list_objects_fsm_v2}], debug).`

There is an EQC test that is added for some pure, helper functions in the new
FSM. There is also a new `riak_test` test that runs the current list objects
test, but uses the new FSM.

* A bunch of duplicated effort in the two FSMs has been put into a shared
  module
* Whether or not to use the new or old FSM is abstracted by the helper module
* The full set of S3 list objects options are supported
Implement `data_size` in multibackend

Port of c582cadccd72208720b0d0f1379e56e2a1119c82 from riak_kv
Fix dialyzer warnings from riak_cs_list_objects_fsm_v2
Handle inclusion of continuations in 2i results
Update to new 2i and CS fold responses
Use new versioned 2i response records

As introduced in riak-erlang-client
40761a7c3e76b6126e196bf0f0fc2c9dceef2c12
Test storing and listing 2000 objects instead of just 100
Update `riakc` and continuation usage
Update comment to reflect reality

We don't test 2000 times yet because erlcloud doesn't do object listing
pagination for us.
Refactor long functions
Fix comment typo
Use new riak_test helper API
Merge pull request #600 from basho/feature/fold-objects-for-list-keys-squashed

Improve object listing performance and efficiency