Remove transient store creation on resource shutdown?

[gdaniel]

Creating a transient store / backend on resource shutdown consumes memory and may be problematic for the maven build, where all the tests are executed in the same JVM, creating many transient (and useless) stores.

[yvrng]

It's a strange behavior, because the transient Store created when closing is empty. The problem may comes from elsewhere.

We should perhaps create a ClosedStoreDecorator to avoid changes to a closed Resource ?

[yvrng]

Fixed in #69: A Store is no longer copied when the associated Resource is unloading: a Resource should not be called if it's not loaded.

This can also help for issue #61.