Rht
RhtRemoval is what Rht.remove reports back.
gcNodes are the tombstones this removal made collectable.
valueDropped is the size of the value the removal stopped charging for, which no node's RhtNode.dataSize accounts for any more. The caller subtracts it from whichever side of the ledger was holding it: live for an attribute that was live on a live node, gc for one on a node that is itself a tombstone. Zero when the attribute was already a tombstone, since a tombstone's value was not being charged in the first place.