Increasing RAM usage over time

A number of devs have observed increasing RAM usage over time, in (at lest) some cases at a very substantial rate.

An easy way to replicate this is to run WCBench[1] against ODL for a while, then use the included `stats.py` script to build a graph of used RAM per run (`./stats.py -g ram`, the RAM data will be collected automatically).

It's possible that this has to do with logging being enabled. It's also possible that this isn't harmful (unused RAM is wasted RAM). I'm not trying to wave a "serious problem" flag, just to document this behavior and hopefully get someone (maybe me when I have time) to give it some attention.

[1]: https://github.com/dfarrell07/wcbench