wireproto: disallow commands handlers for multiple transport versions I think it will be more trouble than it is worth to code version 1 and version 2 command handlers to the same interface. It will feel awkward to shoehorn functionality into e.g. the version 1 protocol handler interface. This would likely constrain the ability for version 2 to evolve. Previous commits introduced a clean separation between command handlers for version 1 and version 2 transports. This commit reinforces that separation by dropping support for having a single command handler service both version 1 and version 2 transports. Differential Revision: https://phab.mercurial-scm.org/D3208