wireprotoserver: handle SSH protocol version 2 upgrade requests This commit teaches the SSH server to recognize the "upgrade" request line that clients send when they wish to switch the channel to version 2 of the SSH protocol. Servers don't honor upgrade requests unless an experimental config option is set. Since the built-in server now supports upgrade requests, our test server to test the handshake has been deleted. Existing tests use the built-in server and their output doesn't change. The upgrade is handled in our state machine. The end result is a bit wonky, as the server transitions back to version 1 state immediately after upgrading. But this will change as soon as version 2 has an actual protocol that differs from version 1. Tests demonstrating that the new server is a bit more strict about the upgrade handshake have been added. Differential Revision: https://phab.mercurial-scm.org/D2204

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