Solution: if all peers of a socket are >= 3.1 use sub/cancel commands
instead of the old 0/1 messages.
For backward compatibility, move the handling of 0/1 or sub/cancel
command strings to the encoders, so that the right thing can be done
depending on the protocol version.
Do not set the command flag until the encoder, so that we can handle
the inproc case (which skips the encoder).
Solution: add some msg helpers to parse commands, and check for
subscribe or cancel commands and process them accordingly in the xpub
and xsub classes.
