wireproto: introduce an abstractserverproto class sshserver and webproto now inherit from an abstractserverproto class. This class is introduced for documentation purpose.

wireproto: document possible return type The wireprotocole command use a small set of class as return value. We document the meaning of each of them. AS my knowledge of wire protocol is fairly shallow, the documentation can probably use improvement. But this is a better than nothing.

pull: add a set of steps that remain to be done during the pull With bundle2 we'll slowly move current steps into a single bundle2 building and call (changegroup, phases, obsmarkers, bookmarks). But we need to keep the old methods around for servers that do not support bundle2. So we introduce this set of steps that remains to be done.

pull: put discovery step in its own function The discovery is a full step that will grow to contain more than changesets. We set it in its own function like all other steps.

pull: move the cgresult logic in _pullchangeset Logical follow up of the previous changeset. The `_pullchangeset` is now responsible of setting the `pullop.cgresult`.

pull: move return code in the pull operation object With bundle2 we'll have multiple code path susceptible to be responsible for adding changeset during the pull. We move it to the pull operation to simplify this the coming logic. The one doing the adding the changegroup will set the value.