pull: move bookmark pulling into its own function This requires adding an attribute on the pulloperation object. The bookmark pulling is protected behind a "todostep" as other steps of pull.

pull: perform the todostep inside functions handling old way of pulling This matches what is done during push.

bookmark: remove now unused `pushtoremote` function Everything is unified in the push discovery now.

push: gather all bookmark decisions together The discovery phases for bookmarks now use the list of explicitly pushed bookmarks to do addition, removal and overwriting. Tests are impacted because this reduces the amount of listkeys calls issued, removes some duplicated messages and improves the accuracy of some messages.