Eliminate the "prev" pointer in the rdataset header.
rollback_node() incorrectly required that node->references == 0. We cannot assume that a node has no references when an update is rolled back. We now mark any rolled back rdatasets with the RDATASET_ATTR_IGNORE attribute. When the node eventually has a zero reference count, IGNOREd rdatasets will be cleaned up. In the meantime, they will be ignored.
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