MavFrame serialize into a vector with wrong fields order

[qwerty19106]

True fields order for Mavlink Serialization:

• magic (start)
• len
• ... (V2 fields)
• sequence
• system_id
• component_id
• message_id
• payload
• checksum

Now (MavFrame::ser method):

• system_id
• component_id
• sequence
• message_id
• payload

In addition, MavFrame::ser and MavFrame::deser methods are not used in the MavConnection implementors as expected.