Clean up methods related to presence

[chacha912]

What this PR does / why we need it:

• Return presence to user by retrieving only online clients and making a copy
• Trigger unwatched event instead of presence-changed event during detach

Which issue(s) this PR fixes:

Related yorkie-team/yorkie-js-sdk#599

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 #594 (27e541c) into main (036a991) will decrease coverage by 0.21%.
Report is 1 commits behind head on main.
The diff coverage is 3.70%.

@@            Coverage Diff             @@
##             main     #594      +/-   ##
==========================================
- Coverage   51.11%   50.90%   -0.21%     
==========================================
  Files          66       66              
  Lines        7008     6991      -17     
==========================================
- Hits         3582     3559      -23     
- Misses       2952     2962      +10     
+ Partials      474      470       -4     

Files Changed	Coverage Δ
api/converter/to_bytes.go	64.07% <ø> (ø)
api/converter/to_pb.go	54.26% <ø> (-0.87%)	⬇️
client/client.go	13.48% <0.00%> (-0.18%)	⬇️
pkg/document/document.go	44.55% <0.00%> (+1.69%)	⬆️
pkg/document/internal_document.go	13.67% <0.00%> (-3.53%)	⬇️
server/rpc/admin_server.go	53.97% <0.00%> (ø)
server/backend/database/memory/database.go	53.13% <100.00%> (ø)
server/backend/database/mongo/client.go	39.41% <100.00%> (ø)

... and 3 files with indirect coverage changes

[hackerwins]

Overall looks good. I left a minor question.