Improving clearCache and mutableObjectFromJSONDataWithParseOptions: performance.

Replacing 1024 memset calls with a single call.

clearCache before:
![clearCache before](http://i.agilebits.com/rk/clearCache_before_173D8050.png)

clearCache after:
![clearCache after](http://i.agilebits.com/rk/clearCache_after_173D801D.png)

mutableObjectFromJSONDataWithParseOptions before
![mutableObjectFromJSONDataWithParseOptions before](http://i.agilebits.com/rk/mutableObjectFromJSONDataWithParseOptions_before_173D8193.png)

mutableObjectFromJSONDataWithParseOptions after
![mutableObjectFromJSONDataWithParseOptions after](http://i.agilebits.com/rk/mutableObjectFromJSONDataWithParseOptions_after_173D80D5.png)