Add close() to Storage

[cinnamon-bun]

What's the problem you want solved?

Some Storage implementations might need to do clean-up operations when closing.

Is there a solution you'd like to recommend?

Add a close() method to the IStorage interface.

[cinnamon-bun]

Done in 6b564a2

    // Close this storage.
    // All functions called after this will throw a StorageIsClosedError,
    // except you can call close() as many times as you want.
    // Once closed, a Storage instance cannot be opened again.
    // TODO: what happens when a long-running process like a sync is happening, and the Storage is closed?
    close() : void;
    // Find out if the storage is closed.
    isClosed() : boolean;