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.
Not sure if it's good idea. User wants first of all rate. E.g. "attitude at 5Hz", not "attitude at 200bytes/s" (try to calculate the rate ;) ). But for some special streams it may be useful as alternative method of configuration, e.g. for FTP / missions / parameters.