Broad 'bulk support'/'performance' discussion

[jsstevenson]

I think we all generally agree that we need to be supporting large/bulk operations, but it's probably too early to crystallize them too much. So, I'll just identify place(s) in this discussion where it should probably be happening, and then we can refine targets later

• HttpAnyVarClient -- support sending lots of vars in one request in put_objects. consider async/aiohttp