eventscript: get rid of ctdb_control_event_script_finished altogether

eventscript: get rid of ctdb_control_event_script_finished altogether

We always have to call it before freeing the state; we should just do
this work in the destructor itself.

Unfortunately, the script state would already be freed by the time
the state destructor is called, so we make the script state a child of
ctdb, and talloc_free() it manually on the one path which doesn't use
the destructor.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>