Reset retransmission times when unpairing own mailbox

When we unpair our own mailbox, any outgoing data we've uploaded to the mailbox (for contacts without mailboxes) will be stranded.

In this case it isn't strictly necessary to reset retransmission timers: the affected contacts don't have mailboxes of their own (otherwise we would have uploaded the data to their mailboxes instead), and if we make a direct connection to an affected contact then any stranded messages will be retransmitted without needing to reset timers, as the ETA via the direct connection will be earlier than the ETA via the mailbox.

However, if we subsequently pair another mailbox then we want any data that's stranded on the old mailbox to be re-uploaded to the new mailbox. So we should reset retransmission timers for contacts who don't have their own mailboxes, either when pairing or when unpairing our own mailbox (as far as I can see we don't need to do both).

Depends on #2173 (closed), #2190 (closed).