ConcurrentHashTable::get_copy requires the CONFIG class to have a proper implementation of notfound(). This is the only use of notfound(). None of the existing CHT uses provide that function. Providing such an implementation requires the CHT's VALUE type to have a copy constructor, and probably requires it to have some distinct sentinal value.

Fortunately, there are no uses of ConcurrentHashTable::get_copy. The simplest thing to do is remove this unneeded "feature".