Add the ability to duplicate a backup cursor

MongoDB has a project to allow cross shard consistent backups, in order to do that they want the ability to "update" the content of a backup cursor to include log files since the backup was first started.

To do that we've decided to implement the ability for users to clone a backup cursor. The cloned cursor would inherit the locks of the parent backup cursor. It would need to be explicitly closed prior to the parent backup cursor being closed.

The secondary backup cursor should only accept the target=log: argument, and return all log files.