Allow webskin tracer to use object broker cached objects

Current webskin tracer behaviour completely disabled cache for both objects and webskins, so it makes it difficult to understand how quickly a webskin will execute on a warm cache.

This change will allow isCacheable to return true for object retrievals when webskin tracer is turned on (e.g. ?tracewebskins=1)

The old behaviour, which doesn't seem terribly useful for performance tuning, can still be observed by also turning off cache in the tray or on the URL (e.g. ?tracewebskins=1&flushcache=1). Note that flushcache is a sticky setting, so if it's turned on then it would need to be turned off again to allow cached objects to be used again (e.g. ?tracewebskins=1&flushcache=0).