push: feed pushoperation object to _pushobsolete function This will allow the obsolete marker synchronisation to have full information on the ongoing push and pass information to other operation.

push: move obsolescence marker exchange in the exchange module The obsolescence marker exchange code was already extracted during a previous cycle. We are moving the extracted functio in this module. This function will read and write data in the `pushoperation` object and I prefer to have all core function collaborating through this object in the same place. This changeset is pure code movement only. Code change for direct consumption of the `pushoperation` object will come later.

push: feed pushoperation object to _pushbookmark function This will able the bookmark synchronisation to have full information on the ongoing push and pass information to other operation.