Filter out unsubscribed documents key in DocEvent

[chacha912]

What this PR does / why we need it:

When firing a DocEvent, instead of directly passing the documentKeys of the published event,
it is passed to the subscribed documentKey.
Previously, an error occurred in the following code when a client that only subscribed to docKey1
received a documentKeys of [docKey1, docKey2].

func (c *Client) Watch(
  ctx context.Context,
  docs ...*document.Document,
) (<-chan WatchResponse, error) {
  // ...
  // handle WatchDocumentsResponse
  handleResponse := func(pbResp *api.WatchDocumentsResponse) (*WatchResponse, error) {
    switch resp := pbResp.Body.(type) {
    case *api.WatchDocumentsResponse_Event:
      eventType, err := converter.FromEventType(resp.Event.Type)

      switch eventType {
      case types.DocumentsWatchedEvent, types.DocumentsUnwatchedEvent, types.PresenceChangedEvent:
        for _, k := range converter.FromDocumentKeys(resp.Event.DocumentKeys) {
           cli, err := converter.FromClient(resp.Event.Publisher)
           // 💣 The document Keys in WatchDocumentsResponse may contain document keys 
           // that the client is not subscribed to.  
           attachment := c.attachments[k.String()]				 
     }
}

Which issue(s) this PR fixes:

Address yorkie-team/yorkie-js-sdk#455

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Additional documentation:

Checklist:

• Added relevant tests or not required
• Didn't break anything

[codecov]

Codecov Report

Merging #463 (faafccc) into main (de20c70) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #463      +/-   ##
==========================================
+ Coverage   46.56%   46.60%   +0.03%     
==========================================
  Files          70       70              
  Lines        5723     5727       +4     
==========================================
+ Hits         2665     2669       +4     
  Misses       2764     2764              
  Partials      294      294              

Impacted Files	Coverage Δ
server/backend/sync/memory/pubsub.go	47.57% <100.00%> (+2.11%)	⬆️

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[hackerwins]

As we discussed, I think it would be nice to add test code like below.

1. clientA attaches doc1
2. clientB attaches doc1
3. clientA attaches doc2. At this time, checks whether information of doc2 is coming to clientB

[chacha912]

@hackerwins Thank you for suggesting a testing procedure.
I added the test as follows:

1. Client A attaches doc1
2. Client A watches doc1
3. Client B attaches doc1 and doc2
4. Client B watches doc1 and doc2. <- At this point, check whether Client A receive the information of doc2

In the original logic, an error occurs at step 4, but the test is passed after modification.

[hackerwins]

Thanks for your contribution.
I left a trivial comment.