Xpra: Ticket #1154: hello packet should use namespace

Follow up from #486. We could send both structured and flat data (structured isn't that much bigger anyway), then the server can unflatten stuff and move to using structured data everywhere. Then at some point in the future, we can remove support for flat hello packets.

Sat, 23 Jan 2021 05:16:28 GMT - migration script:

this ticket has been moved to: https://github.com/Xpra-org/xpra/issues/1154