Implement the pushOnly flag of the pushpull API

[humdrum]

What this PR does / why we need it:

• Extent PushPull API by adding a pushOnly flag to support Client Sync mode. (Add Sync mode per documents #482)
• If pushOnly is true, Server makes a push-pull response with empty changes unless data has been changed.
• If a push-pull response has changes that have already been performed by local changes on the client side. Send empty operations to prevent redoing those operations.
  • Due to that modifying operations, Deep coping of chageInfo is needed.

Which issue(s) this PR fixes:

Fixes #482 (Server side only)

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

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #500 (a5c9cdf) into main (6f9c5ad) will decrease coverage by 0.12%.
The diff coverage is 24.00%.

❗ Current head a5c9cdf differs from pull request most recent head dfd4404. Consider uploading reports for the commit dfd4404 to get more accurate results

@@            Coverage Diff             @@
##             main     #500      +/-   ##
==========================================
- Coverage   50.43%   50.31%   -0.12%     
==========================================
  Files          64       64              
  Lines        5645     5660      +15     
==========================================
+ Hits         2847     2848       +1     
- Misses       2457     2470      +13     
- Partials      341      342       +1     

Impacted Files	Coverage Δ
client/client.go	15.38% <0.00%> (-0.36%)	⬇️
server/backend/database/change_info.go	24.13% <0.00%> (-5.03%)	⬇️
server/rpc/yorkie_server.go	38.60% <71.42%> (-0.06%)	⬇️
server/backend/database/memory/database.go	52.57% <100.00%> (ø)

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[hackerwins]

Thanks for your contribution.

Even though it was the first PR, the operation was a bit complicated, but thank you for working on it.