KAFKA-428 bug where copy existing is not persisted after it has finished copying

[rahulbats]

…sk switched to change stream

the isCopying flag tracks wether the connector task is copying collection data or tracking change stream. This flag needs to be persisted, so that if the connector restarts later it does no start copying the collection again. There was a if statement which persisted this flag only when iscopying is true which is wrong, as the iscopying flag needs to be persisted in offsets after it has turned false when it has finished copying