lnmsg decode/encode message needs rewrite; to support TLVs

[SomberNight]

• lightning.json should be updated to reflect current BOLTs
  • It was generated by a script (structured.py), that @ysangkok wrote and got upstreamed. It worked by processing the output of another script extract-formats.py, which in turn processes the BOLT documents directly. However the BOLTs and the output of extract-formats.py seemingly changed a lot in the meantime and structured.py is now broken and unmaintained.
  • We should either fix structured.py, or rather, we could use extract-formats.py directly. This script looks to be actively maintained as the BOLTs change, by Rusty.
• Accordingly, lnmsg.py will need to be rewritten.
• Note that this is a requirement of implementing TLV support.
• btw, ChannelDB.load_data() became significantly slower when we changed to storing raw messages in the db. Most of the time is spent in lnmsg.decode_msg. I believe lnmsg.decode_msg could be made faster, perhaps even as-is, but there is no point now if it will need significant changes anyway.