Fill in missing methods in ManagedMap

[ryanrdoherty]

ManagedMap is a convenience java.util.Map interface over the InMemoryCache implementation. We're now using it in the EDA data service in some plugins. Because we would like to be able to completely clear those caches, we need the void clear() method to be implemented. Since it's easy, filling in boolean hasKey(key) with return get(key) != null would be nice. If other methods are easy to fill in, maybe do those too for a more complete Map implementation, but definitely those two.