Allow retransmission if it will result in faster delivery

Each time a message is sent to a contact, the message's send time is updated based on the maximum latency of the transport, and the message cannot be sent to the contact again until the send time is reached. This prevents messages from being retransmitted unnecessarily while they're still in transit. However, if a message is in transit over a high-latency transport, it may be desirable to allow it to be retransmitted over a low-latency transport to allow faster delivery.

Instead of comparing the send time to the current time, it should be compared to the expected delivery time over the current transport. The current time plus the maximum latency of the current transport can be used as a first estimate of the expected delivery time; we can get clever with round-trip time measurements later.