wireprotoserver: split ssh protocol handler and server
We want to formalize the interface for protocol handlers. Today,
server functionality (which is domain specific) is interleaved
with protocol handling functionality (which conforms to a generic
interface) in the sshserver class.
This commit splits the ssh protocol handling code out of the
sshserver class.
Differential Revision: https://phab.mercurial-scm.org/D2080