Smarter handling of cached resources

[ksons]

Currently the XHR request gets cached as it could be reused in structured file formats that allow to adress multiple objects within one resource. However, the cached responses claim a lot of memory.

The desired behaviour:

1. Don't cache responses for unstructured file formats (e.g. OpenCTM, xml3d-json)
2. Add heuristic that manages freeing the response for structured resources. A simple one could be to free the resources once there is no request in the queue that requires the resource. Another options include using a LRU or LFU strategy

[ksons]

Since 4.5, the resource manager does not cache the raw XHR response but the processed data in whatever the format handler thinks is most suitable. This saves a lot of memory, particular for JSON and XML request, where the XHR is quite memory consuming.

However, unused resources are not yet detected and remain cached. I opened Issue #27 for this.