Mavlink parameter sync and general congestion review

[AuterionWrikeBot]

Mavlink parameter sync ignores the mavlink instance data rate and current rate multiplier (throttling). It does check the tx buffer, but this isn't sufficient. As a result some setups (eg SiK radios without flow control) can be unusable at times failing to complete the initial parameter sync. #10394 (comment)

Ideas to explore

• parameter sync respect data rate and rate multiplier
  • move back to TX thread? @bkueng
• pause all other telemetry (except HEARTBEAT) during sync?
• rate multiplier consider additive increase, multiplicative decrease
  • mavlink data rate start at minimum and increase
• rate multiplier treat RADIO_STATUS rxerrors as critical and drop the rate substantially
• mavlink track and send drop rate for rate throttling (MAVLink Status mavlink/qgroundcontrol#6847)
  • add new mavlink status message to send drop rate (@dogmaphobic)
  • would also help on bad UDP connections
• QGC request data streams that are actually required.
• for example default to Mavlink MINIMAL (https://github.com/PX4/Firmware/blob/master/src/modules/mavlink/mavlink_main.cpp#L1882) and only add new streams or increase rates as needed.
• Mavlink NORMAL is roughly 100x more data than MINIMAL and just as functional for the vast majority of usage

Parameter sync is the best target to improve, but all of this should apply to mission sync or FTP as well.

[dagar]

@thomasgubler another duplicate from the bot - #10618