Enhancement: add failover master link

Currently, if two --master links are defined, both mavlink streams are merged, causing mavlink sequence errors on the receiving end (ie, Mission Planner). This results in erroneous link quality calculations. Proposed is a --failover link category that is used when heartbeat is dropped from the --master link, and switches back over to --master when heartbeat is regained.

The proposed implementation of this feature is to enable a high speed primary link and longer range backup link, or potentially cellular backup to a primary RF link.

Start by tracing how MAVProxy parses and manages the --master links, then inspect the heartbeat handling and MAVLink stream forwarding paths. Define the failover behavior around lost and regained heartbeats, and consider how to test that primary and backup links do not merge streams and cause sequence errors.

Written by the indexing model from the issue text.