Reduce needless copies in the Prozess consumer

I could never get the performance of Prozess up to what's needed for the topics I'm subscribing to, so I ended up writing [an adapter](https://github.com/uber/kafka-spraynozzle) to solve my particular situation, but it would still be a waste to not have these perf improvements in Prozess so others don't have to make such a leap for a while (or at all, if they have a more predictable source stream).