push: use bundle2 to push phases when available We now use a bundle2 container to push all phase updates at the same time. This is a significant step forward, even if further refactoring is needed to unify phase push with the changeset push.

pushkey: add an ``encode`` function This function is just a shorthand for ``encoding.fromlocal``. It will help hiding the encoding business from other code exchanging pushkey data over the wire.

bundle2: add ``pushkey`` support After ``listkeys`` we can now include ``pushkey`` request in a bundle2. The part uses a very simple scheme closest as possible to the current wireproto command for ``pushkey``. We may eventually decide for a more sophisticated part format before the protocol becomes final.