It was needed when extracting data from a domain tree chain. The chain
now can hold mutable pointers too, so we use that (and some amount of
template bureaucracy) to avoid the cast.

While the interface changed (on the core find function, it is not
possible to pass const node chain and have a mutable node get out), it
doesn't seem to influence the current code. Also, it is a private
interface anyway, so it should be safe.