wireprotoserver: extract SSH response handling functions The lookup/dispatch table was cute. But it isn't needed. Future refactors will benefit from the handlers for individual response types living outside the class. As part of this, I snuck in a change that changes a type compare from str to bytes. This has no effect on Python 2. But it might make Python 3 a bit happier. Differential Revision: https://phab.mercurial-scm.org/D2091