In preparation for removing in-memory invoice objects.
Since they will be removed, there will no longer be any
individual invoice objects to attach to.
We usually did this, but sometimes they were named after what they did,
rather than what they cleaned up.
There are still a few exceptions:
1. I didn't bother creating destroy_xxx wrappers for htable routines
which already existed.
2. Sometimes destructors really are used for side-effects (eg. to simply
mark that something was freed): these are clearer with boutique names.
3. Generally destructors are static, but they don't need to be: in some
cases we attach a destructor then remove it later, or only attach
to *some* cases. These are best with qualifiers in the destroy_<type>
name.
Suggested-by: @ZmnSCPxj
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Error code is inverted (which makes sense: who returns 'true' on
error?), and anyway there's a leak if we do error.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
`pay_index` has no valid value if not PAID anyway, so
we should correctly leave it uninitialized.
Analysis via valgrind will catch incorrect use of
uninitialized fields.
If we load it with a dummy 0 value, then an
incorrect use of `pay_index` whan invoice is not
PAID will not get caught by valgrind.
