This is a preparatory pull to try to make reviewing #6557 easier.
In #6557, I added tracking packages of transactions to the mempool (for each tx, we track all the “descendant” transactions that depend on that tx), in order to make the mempool limiting code more effective. This PR is a standalone implementation of mempool descendant tracking, including the policy limits on transaction chains (limiting ancestors and descendants) proposed in #6557.
I’ll rebase that pull off these commits assuming we can agree on this approach.