Reset retransmission times when a contact's mailbox properties change

When a contact starts or stops advertising a mailbox, data may be stranded on a mailbox that will no longer be checked. Specifically:

• When a contact starts advertising a mailbox they'll stop checking our mailbox. Any outgoing data we've uploaded to our own mailbox will be stranded. When we receive the contact's new transport properties that advertise the mailbox, we should reset the retransmission times of any unacked messages so they can be uploaded to the contact's mailbox
• When a contact stops advertising a mailbox, any outgoing data we've uploaded to the contact's mailbox may be stranded. When we receive the contact's new transport properties that stop advertising the mailbox, we should reset the retransmission times of any unacked messages so they can be uploaded to our mailbox

Similarly, if a contact's mailbox properties change, indicating that the mailbox has been replaced (rather than added or removed), then we should reset retransmission timers for that contact so that any data stranded on their old mailbox is re-uploaded to the new one.

Depends on #2181 (closed), #2190 (closed).