Lock watchDocuments depending on the client and doc

[chacha912]

What this PR does / why we need it:

Since the client cannot wait for the watch stream to be disconnected,
add a lock to re-establish the watch stream after it has been disconnected.

Which issue(s) this PR fixes:

Related #479

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 #506 (9454952) into main (bbd06b2) will decrease coverage by 0.10%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main     #506      +/-   ##
==========================================
- Coverage   50.31%   50.22%   -0.10%     
==========================================
  Files          64       64              
  Lines        5660     5671      +11     
==========================================
  Hits         2848     2848              
- Misses       2470     2481      +11     
  Partials      342      342              

Impacted Files	Coverage Δ
server/rpc/yorkie_server.go	37.30% <0.00%> (-1.30%)	⬇️

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[hackerwins]

Thanks for your contribution.