Exercise erlang http client 2i

These are some changes to the 2i tests to exercise new 2i support in the erlang http client and take advantage of its HTTP streaming support specially. The tests have their own implementation of http streams fetching that is a bit of a maintenance headache.

More changes are needed. I'm planning on adding non-streaming support for 2i operations to the client and use that instead in the tests, among other things.

/cc @seancribbs @russelldb