keeping private state in each backend that has inserted and deleted the same
tuple during its current top-level transaction.  This is sufficient since
there is no need to be able to determine the cmin/cmax from any other
transaction.  This gets us back down to 23-byte headers, removing a penalty
paid in 8.0 to support subtransactions.  Patch by Heikki Linnakangas, with
minor revisions by moi, following a design hashed out awhile back on the
pghackers list.