We've been slack, but it's going to be important for testing
ratelimiting. And it currently has a minor memory leak.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Rather than reaching into data structures, let them register their own
callbacks. This avoids us having to expose "memleak_remove_xxx"
functions, and call them manually.
Under the hood, this is done by having a specially-named tal child of
the thing we want to assist, containing the callback.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>