eventscript: fix callback after free

eventscript: fix callback after free

ctdb_event_script_callback() takes a mem_ctx arg which it doesn't use, but
the implication is pretty clear, that when that mem_ctx is freed, the callback
shouldn't happen.  Indeed, Ronnie reproduced a case where that callback
refers to freed memory, in the ip reallocation code under stress.

So attach the callback to the mem_ctx they give us, and remove it from the
script state structure when that's freed.  It's a bit weird, but it works.

CQ: S1026179
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>