Check own mailbox when coming online
Check our mailbox for incoming data when coming online. Keep checking periodically until our direct hidden service has been available for some overlap period, to avoid races where the contact fails to connect to our hidden service and uploads data to our mailbox instead.
We should also check periodically (perhaps with a longer interval) while we're online, so that we can warn the user if the mailbox becomes unreachable.
Depends on #2170 (closed), #2171 (closed).