wireprotoserver: make some instance attributes private
sshserver attempts to conform to a well-defined interface. The instance
attributes changed as part of this commit don't appear to be part of
that interface. So prefix them with _ to mark them as private.
Differential Revision: https://phab.mercurial-scm.org/D1988
Hello, Foobar
['Hello, Fo, =;:<o', 'Hello, Bar']