Separate the protocol version number from the client API version macros.
They're not meant to be related concepts.
The present value of current_protocol_version is unchanged by this.
And hopefully it's now more clear to the reader what the range of
allowable versions is.
. Fixes: https://bugs.launchpad.net/bugs/1507982.
Approved by PS Jenkins bot, mir-ci-bot, Kevin DuBois.