Implementing an OnSubscribe that emits a Cursor as it moves to next.

This commit removes CursorIterableAdapter.java and implements the
Cursor navigation and safe closing in one place.

The option to use doOnTerminate(..) will not work as well to close
the Cursor since downstream operators while emiting errors would
not pass through the operator closing the Cursor.

Signed-off-by: David Marques <dpsmarques@gmail.com>