Make retransmissions in the sync protocol more flexible

This branch adds an eager retransmission mode to the sync protocol. In eager mode, all messages that have not been acked by the contact are sent, regardless of whether they're due for retransmission.

Eager mode is used when the transport plugin is "lossy and cheap", meaning that the transport is likely to lose streams and the cost of retransmitting data is low. Removable drives are the only lossy and cheap transport so far.

Closes #2045 (closed)