bookmark: introduce a 'applychanges' function to gather bookmark movement
We want to track bookmark movement within a transaction. For this we need a
more centralized way to update bookmarks.
For this purpose we introduce a new 'applychanges' method that apply a list of
changes encoded as '(name, node)'. We'll cover all bookmark updating code to
this new method in later changesets and add bookmark move in the transaction
when all will be migrated.
obsstore: keep self._data updated with _addmarkers
This makes sure obsstore._data is still correct with added markers.
The '_data' propertycache was added in
17ce57b7873f.