missed features

[Xudong-Huang]

here list some features that we are not supported and may implement in future

• only support payment definition
• not support private payment
• prepare catch up units for other hubs
• witness list is now fixed
• unit content_hash is empty (always a good unit)
• definition only support sig data_feed text
• not support multi sign
• not support wallet logic (composer)
• not support network self connection detection

[Xudong-Huang]

definition is a big independent feature for token platform, need to address in the other milestone