Optimize sync requests in push-only syncMode to avoid unnecessary syncs

[chacha912]

What happened:

Sync requests are being triggered by remote changes even when syncmode is set to pushonly and there are no local changes.

What you expected to happen:

When syncmode is set to pushonly and there are no local changes, sync requests should not be triggered by remote changes.

How to reproduce it (as minimally and precisely as possible):

1. clientA: Set syncmode to pushonly.
2. clientA: Ensure there are no local changes.
3. clientB: Edit document.
4. clientA: Observe that a sync request is still triggered.

Anything else we need to know?:

Environment:

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