wireprotoserver: move SSH server operation to a standalone function The server-side processing logic will soon get a bit more complicated in order to handle protocol switches. We will use a state machine to help make the transitions clearer. To prepare for this, we move SSH server operation into a standalone function. We structure it as a very simple state machine. It only has two states for now, with one state containing the bulk of the logic. But things will evolve shortly. Differential Revision: https://phab.mercurial-scm.org/D2203