Allow to disable the render cache

On a project my firm is involved with we are using render cache to save on PHP memory overhead (works great btw). The problem is there doesn't seem to be any inbuilt mechanism in core to turn render caching off. This is very inconvenient for local development. I would also imagine that there might be other use cases where it would be nice to able to turn it off. It seems the only way to accomplish this currently is to wrap the addition of '#cache' to render arrays in a custom procedural helper. The patch I'm proposing adds a render_cache check in drupal render before sending the output of an array to cache_set/cache_get. The ideal solution would create a variable that could be set to FALSE but which defaults to TRUE in order to maintain backwards compatibility.