Handle presence updates before attaching a document

[chacha912]

Description:

While it is possible to edit the document.root without attaching a document (with the default actor ID set to 000000000000000000000000), the presence changes is not working as expected. Although a presence-changed event occurs, the presence value is undefined, and user cannot get presence since document is not in an online status. Let's address this case for users who attempt editing before attaching.

const client = new yorkie.Client(testRPCAddr);
await client.activate();
const doc = new yorkie.Document<DocumentType, PresenceType>(docKey);
doc.subscribe('presence', (event) => { console.log('d1 event', event); });

// update presence
doc.update((root, p) => {
  root.k1 = 'v1';
  p.set({ name: 'a' });
});

// and then attach document 
await client.attach(doc, { initialPresence: { name: 'A' } });

Considerations:

• Is it possible to ignore the initialPresence value when a presence value already exists?
• Before attachment, the user can be considered offline. Current methods only retrieve presence for online clients, so we may need to introduce a method to retrieve presence including offline clients.

Why: Users can update presence before attaching a document.