config: fix restoreconfig of non existing config When the section, but no value existed, the `del` call raised a key error.

push: use stepsdone for obsmarkers push We do not have infrastructure to include obsolescence markers in the bundle2 push from core. But extensions may so we make sure it would not be sent twice.

push: introduce a discovery step for obsmarker The discovery step is still not doing anything smart. But this will allow extension to wrap it.

push: move the list of obsmarker to push into the push operation The list is now carried in the push operation, this will let extensions override it.