Resurrect tw_memory?

[JohnPJenkins]

tw_memory is currently disabled by default and it's unclear what level of support there is for it, especially since the SR refactor/merge.

In CODES, our modelnet and local storage models currently piggyback messages on top of modelnet/lsm-specific messages to provide an abstraction over IO/communication. That means the per-event memory we allocate is approx 2x larger than it needs to be! Enabling the passing of event data through tw_memory functionality would greatly improve the memory characteristics and likely performance of our models, not to mention make our current piggybacking mechanism less hairy.