Aborted introduction sessions can't be retried
While working on #2079 (closed) I discovered that if an introduction session is aborted while handling an incoming message, subsequent attempts to retry the introduction (with the same introducer and introducees) will fail.
This happens because the sender records the ID of the message that triggered the abort in the session, but the recipient doesn't. Any subsequent message with the same sender and recipient is rejected by the recipient because it depends on the message that triggered the abort, not on that message's predecessor as the recipient expects.