These are standalone fixes that came out of my debugging Zygo's problems. The first two address a problem with how we handle restarting relocation. Previously this rarely happened, because if it had people would have complained. The restart logic was broken in a few subtle ways, and these two patches address those issues. The third patch just boggles my mind. We were recording reloc roots based on their current bytenr. This worked fine if we never restarted, but broke if we had to lookup a ref to a reloc root that we found on the tree. This is because that would point at the commit root of the reloc root, but if we had modified the reloc root we'd no longer be able to find it. And finally the last one was a weird deadlock that Zygo's insane test rig found, as he runs the dedup thing while balancing and deleting snapshots, which made this thing fall out. Thanks, Josef
