NodeDocumentCache#get method should be synchronized

The NodeDocumentCache#get(String key, Callable) method may modify the cache if there's no value for the given key. Therefore, it should be synchronized using NodeDocumentLocks, as all the methods modifying the cache are.

Right now, the get() method is invoked in 3 different places within the Oak codebase. All of them are critical sections anyway, so this modification won't result in a performance decrease.