Improve automerge sync

[nikgraf]

• generate automergeId from spaceId
• Store the latest server doc separate so that the latest changes can be determined
• Calculate the diff changes and send them in the message instead of simply the latests change (this should also prevent sending an update after receiving it from a real time update)
• Activate automerge-repo-indexeddb for storing the document locally - investigate how loadingDone needs to change

(ideally solved by integrating KeyHive)