Fix memory leak in Util#generateModel(...)

[bnouwt]

The MetadataKB and derived classes (StorageKB and PushpullKB) make use of the Util#generateModel(...) method to combine a graph pattern with a binding set to get RDF data. This method uses a cache to increase the performance, but from analyzing a memory dump it seems that this cache causes high memory usage of the StorageKB and PushpullKB.

[bnouwt]

It seems like part of the problem is that the cache of the above method is 1 hour and if this is being used a lot (like with StorageKB and PushpullKB) it will keep all data (it stores all subjects/predicates and objects) in cache for 1 hour. Not sure if this explain the multi gigabyte memory usage, but we'll see if we reduce it to something like 10 minutes.