Description: The upgrade tool dn2rdn shares the reindex code, in
which a cursor is set to each entry in id2entry.db# converting
dn to rdn and put it into the btree with db->put one by one.
It turned out the strategy does not work for the case the btree
structure is modified. In that case, the btree could get corrupted.
This patch creates a new id2entry_tmp.db# from the scratch. Once
all the entries are successfully converted, the old db is removed
and id2entry_tmp.db# is renamed to id2entry.db#.