Wire private messaging through the client:
- MessageFactory::build_message_user (code 22) and build_message_acked
(code 23);
- the incoming MessageUser handler now acknowledges freshly delivered
messages and forwards them to the client instead of only logging them;
- ClientContext buffers received UserMessages;
- Client::send_private_message and Client::take_private_messages expose the
feature, and UserMessage (with accessors) is re-exported from the crate.
Verified end to end against soulfind: a_private_message_is_delivered_between_users
logs in two clients and asserts one receives the other's message.
michel
a7372cc88e