Resolve concurrent map issue

[chacha912]

What this PR does / why we need it:

The subscriptionMapByDocID was being accessed and utilized without proper mutex protection.
As a result, a fatal error occurred when simultaneous reads and writes were performed.
To resolve this issue, I added the mutex lock when iterating the map.

Which issue(s) this PR fixes:

Fixes fatal error

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 #538 (f8a1576) into main (dbcec29) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #538      +/-   ##
==========================================
+ Coverage   50.92%   50.95%   +0.03%     
==========================================
  Files          65       65              
  Lines        5978     5982       +4     
==========================================
+ Hits         3044     3048       +4     
  Misses       2539     2539              
  Partials      395      395              

Impacted Files	Coverage Δ
server/backend/sync/memory/coordinator.go	27.58% <100.00%> (-2.42%)	⬇️
server/backend/sync/memory/pubsub.go	44.21% <100.00%> (+3.09%)	⬆️

[hackerwins]

Thanks for your contribution. 👍