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