Close objects contained inside an ObjectCache

As a plugin author, I often need a place where to store an object which should be a singleton, e.g., at CoreContainer-level. To do so, in spite of a better place, I use the ObjectCache as a holder of my singleton. This makes it easy to store and retrieve my singleton, but it is quite complex to clean it up when CoreContainer is shutting down.

My proposal is hence to have ObjectCache to automatically close any Closeable object that is stored within it, in order to serve this use case for plugin authors.