Mavlink rates

This PR is WIP. Adds a decent method of printing the radio status.
Yes, I seen. For parameters message size is known, but not for mission messages, so your approach will fail for missions.

What about throughtput, you are right, it may increase it, but only on fast links. 3DR radio with default settings has effective bandwidth ~1...1.5 kbytes/s, that is 5 times less than UART speed. Using large buffer (~10 times larger than radio buffer) will NOT increase throughput at all, but increase latency significantly.

Also idea of sending message only if it fits looks simple and correct but in practice you give much more priority to small messages, and never sending large messages, because smaller ones occupate buffer faster than it freed by radio. I don't think that it's correct.

So what we can do to maximize throughput is to use max mavlink packet size as TX_BUF_MIN_FREE.