New tbswap mode 'update'. (a.k.a. 'reswap') Re-assigns experiment, fixing already assigned nodes in place; tries not to reboot nodes. Doesn't clear port counters, restart event system, etc. A few more things remain to be considered for 'general' use adding new nodes to experiments and modifying topologies, but for replacing failed nodes in experiments or removing virt_nodes from experiments, should work fine.