Bug when performing multiple `sync()`

[chacha912]

What happened: When performing multiple 'pushpull' requests, the SDK doesn't work correctly. For example, the following test case fails: https://github.com/yorkie-team/yorkie-js-sdk/blob/394c061cdac97d47508fee8178ab64874641a580/test/integration/document_test.ts#L57-L93

Anything else we need to know?:

In the test code, four changes are generated:

• change (serverSeq 1): d1 update root['k1'] = [1, 2]
• change (serverSeq 2): c1 initial presence (within c1.attach)
• change (serverSeq 3): c2 initial presence (within c2.attach)
• change (serverSeq 4): d1 update root['k1'].push(3)

When c2.sync() is called, it sends two pushpull requests with checkpoint{clientSeq: 1, serverSeq: 3}. The server responds with change (serverSeq 4), then c2 applies the same change twice, resulting in [1, 2, 3, 3] not [1, 2, 3]. (When multiple changes are pushed, the server handles it to prevent duplicate changes from being stored. However, the client needs to handle it when processing the response.)

I think one of the solution is enqueuing 'sync' requests. The next request should be sent after receiving the response to update the server sequence.

Environment:

• Operating system:
• Browser and version:
• Yorkie version (use yorkie version): 0.4.5
• Yorkie JS SDK version: 0.4.5