MemoryDBProvider.clone()method

Clone this provider into a new plain MemoryDBProvider containing the same items.

• Shallow: new tables and maps sharing the same (immutable) item instances, so cloning is cheap and unchanged items keep their identity.
• The clone is always a plain MemoryDBProvider with plain MemoryTables, even when called on a subclass — writes to the clone only touch its own memory, never a subclass's backing store (e.g. StorageDBProvider persistence).