fix(pubsub): resolve data race in doProduce

[ngjaying]

Fix data race in memory pubsub manager where doProduce accessed the pubTopics map without a read lock.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.90%. Comparing base (bdbe56a) to head (8513189).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3994      +/-   ##
==========================================
- Coverage   71.92%   71.90%   -0.01%     
==========================================
  Files         457      457              
  Lines       52717    52717              
==========================================
- Hits        37913    37906       -7     
- Misses      11877    11887      +10     
+ Partials     2927     2924       -3     

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