performance problems

[D063520]

The newest version of Wikibase sync privilages local changes:

1. local additions will not be deleted
2. local deletion will not be added
   to achieve this the revision history must be analyzed which can be time consuming. This is the case currently when:
3. many statements are added or removed
4. the entity has many statements
5. the revision histroy is long
   One needs to optimize the code. One option is to not scan the whole revision history but do this in a tree search manner