commandserver: add experimental option to use separate message channel This is loosely based on the idea of the TortoiseHg's pipeui extension, which attaches ui.label to message text so the command-server client can capture prompt text, for example. https://bitbucket.org/tortoisehg/thg/src/4.7.2/tortoisehg/util/pipeui.py I was thinking that this functionality could be generalized to templating, but changed mind as doing template stuff would be unnecessarily complex. It's merely a status message, a simple serialization option should suffice. Since this slightly changes the command-server protocol, it's gated by a config knob. If the config is enabled, and if it's supported by the server, "message-encoding: <name>" is advertised so the client can stop parsing 'o'/'e' channel data and read encoded messages from the 'm' channel. As we might add new message encodings in future releases, client can specify a list of encoding names in preferred order. This patch includes 'cbor' encoding as example. Perhaps, 'json' should be supported as well.