pull: 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 `pulloperation` 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 `pulloperation` object will come later.

pull: move `force` argument into pull object One more step toward a more modular pulh function.

pull: move `heads` argument into pull object One more step toward a more modular pull function.