Pointed out by @ShahanaFarooqui, we leave a single unused entry in the datastore,
so we should clean that up too.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
If they have invalid runes, we bail, but if they have runes which used
a different master secret (old commando.py allowed you to override
secret), we just complain and delete them.
Note that this requires more mocks in wallet/test/run-db.c...
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
