Mavlink: don't use mavlink helpers

As discussed here: https://github.com/PX4/Firmware/pull/1212

@LorenzMeier, take a look please.

WIP, if you are happy with approach I will fix all messages in mavlink_messages.cpp, now only hearbeat fixed to use new API.
In the next iteration we should express this explicitly as bandwidth and not try to hit bandwidth with the right rate.