Shuffle messages less in the producer.

[eapache]

Put them in a map right up front in the aggregator, it only requires tracking
one exta piece of metadata (total messages in the map) and it means we don't
have to shuffle them into this form before constructing the request anyways.

One piece of #433.

Not sure if this is worth landing as-is or if it makes more sense to just rewrite the whole thing at once rather than incrementally.

[wvanbergen]

This looks innocent enough to me, but it's hard to look at this wad of code and make sure no changes in behaviour were introduced. Not sure what we can do to overcome that though.

[wvanbergen]

Definitely in favor of doing this one step at a time. After landing this, the second batch of changes to produce requests incrementally will be a lot easier to review.