Improving clearCache performance.

Replacing 1024 memset calls with a single call.

clearCache before:
http://i.agilebits.com/rk/clearCache_before_173D8050.png

clearCache after:
http://i.agilebits.com/rk/clearCache_after_173D801D.png

mutableObjectFromJSONDataWithParseOptions before
http://i.agilebits.com/rk/mutableObjectFromJSONDataWithParseOptions_before_173D8193.png

mutableObjectFromJSONDataWithParseOptions after
http://i.agilebits.com/rk/mutableObjectFromJSONDataWithParseOptions_after_173D80D5.png