Don't use virtual destructor in addrmap

The addrmap polymorphism is sort of "phony" in that there isn't really
code in the tree that can be presented with either type.  I haven't
tried to fix this (though perhaps I may); but meanwhile it's handy for
the next patch if addrmap_fixed has a trivial destructor.  This patch
achieves this by making the addrmap destructor non-virtual, and also
making it protected so that objects of any of these types cannot be
destroyed when only the base class is known.