FetchManager Cache should not pin down entries in memory

[tmagrino]

Right now, the cache used by the dissemination layer, Cache, does not use soft or weak references. This creates a form of memory leak for workers, particularly when using the DummyFetchManager.

Ideally a solution would ensure that entries that the worker is actually using (ie. objects present in the Worker's ObjectCache) aren't evicted due to memory pressure prematurely.

[tmagrino]

As of cc9b255 this only affects the Pastry backed dissemination layer.

[tmagrino]

@liujed points out I was incorrect in my understanding of this part of the code and we're not actually pinning objects in memory. That said, memory pressure is now avoided when it's "dead-ish" weight with cc9b255.