bookmark: introduce a 'bookmarks' part This part can carry and apply bookmarks information. We start with adding the core behavior of the part. In its current form, the part is only suitable for push since it plain update the bookmark without consideration for the local state. Support of the behavior needed for pulling will be added in later changesets.

push: include a 'check:bookmarks' part when possible Before updating the actual bookmark update, we can start with updating the way we check for push race. Checking bookmarks state earlier is useful even if we still use pushkey. Aborting before the changegroup is added can save a lot of time.

bookmark: add a 'check:bookmarks' bundle2 part This part checks that bookmarks are still at the node they are expected to be. This allows a pushing client to detect push race where the repository was updated between the time it discovered the server state and the time it managed to finish its push. Such checking already exists when pushing bookmark through pushkey. This new part can be inserted at the beginning of the bundle, triggering abort earlier. In addition, we would like to move away from pushey to push bookmark. A step useful to solve issue5165.