remove the global rrl_cb_called from the libctdb example

remove the global rrl_cb_called from the libctdb example
and psss it through the callback via private_data.

add a comment that the callback may sometimes have already been invoked
when the ctdb_readrecordlock_async() call returns
and that the application can use *private_data IF the application
needs to know if the callback has already triggered or not.