Replace ETA with max latency in retransmission logic

The sync protocol allows a message to be retransmitted if either the message's send time (also called expiry time in the database code) has been reached, or if the message's ETA via the currently available transport would be earlier than the ETA of the previous copy. The ETA is based on the max latency of the transport.

This second (ETA) condition is met when the previous copy was sent via a higher-latency transport and a lower-latency transport is now available. But this logic has a weird edge case: immediately after sending a message via a higher-latency transport, the message can be sent via a lower-latency transport, as intended. But as the ETA of the first copy approaches, the message stops being sendable via the lower-latency transport.

This edge case is unlikely to matter when the lower latency is a tiny fraction of the higher latency (eg 30 seconds for Tor vs 28 days for removable drives). But it may become important when the lower latency is a significant fraction of the higher latency (eg 14 days for mailboxes vs 28 days for removable drives).

To remove the edge case we should store the max latency of the transport rather than the ETA, and allow the message to be retransmitted if either the send time has been reached (as now), or if the max latency of the currently available transport is less than the max latency of the transport used for the previous copy. This will require a DB migration.