This is to track the regression introduced in master, which needs fixing for 0.14. See #9240, #9371, etc.
* Note: the term "descendant" refers to in-mempool transactions that depend on
 * this one, while "ancestor" refers to in-mempool transactions that a given
 * transaction depends on.
 *
 * In order for the feerate sort to remain correct, we must update transactions
 * in the mempool when new descendants arrive.  To facilitate this, we track
 * the set of in-mempool direct parents and direct children in mapLinks.  Within
 * each CTxMemPoolEntry, we track the size and fees of all descendants.
 *
 * Usually when a new transaction is added to the mempool, it has no in-mempool
 * children (because any such children would be an orphan).  So in
 * addUnchecked(), we:
 * - update a new entry's setMemPoolParents to include all in-mempool parents
 * - update the new entry's direct parents to include the new tx as a child
 * - update all ancestors of the transaction to include the new tx's size/fee
 *
 * When a transaction is removed from the mempool, we must:
 * - update all in-mempool parents to not track the tx in setMemPoolChildren
 * - update all ancestors to not include the tx's size/fees in descendant state
 * - update all in-mempool children to not include it as a parent
 *
 * These happen in UpdateForRemoveFromMempool().  (Note that when removing a
 * transaction along with its descendants, we must calculate that set of
 * transactions to be removed before doing the removal, or else the mempool can
 * be in an inconsistent state where it's impossible to walk the ancestors of
 * a transaction.)
 *
 * In the event of a reorg, the assumption that a newly added tx has no
 * in-mempool children is false.  In particular, the mempool is in an
 * inconsistent state while new transactions are being added, because there may