Remove unknown offline nodes with conflicting unames

I've been trying different solutions. Finally, I feel it still seems better to do this in crm_get_peer(). Please help take a look whether it makes sense, in case I'm doing anything insane :)  We'll continue testing it.

BTW, I noticed attrd doesn't remove cached node even if executing "crm_node --remove".  It seems safe to just remove offline nodes from node cache, right?