Resolve deadlock in subscription registry

[jamesagnew]

No description provided.

[github-actions]

Formatting check succeeded!

[tadgh]

Looks like you moved the synchronizing over into the map, and removed synchronization on the cache itself? I don't understand why this fixes it, but I assume you do!

[fil512]

Nice!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.50%. Comparing base (406db33) to head (5b9d24f).
Report is 169 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #6632      +/-   ##
============================================
- Coverage     83.54%   83.50%   -0.04%     
- Complexity    27432    28564    +1132     
============================================
  Files          1707     1797      +90     
  Lines        106185   111161    +4976     
  Branches      13397    13965     +568     
============================================
+ Hits          88710    92826    +4116     
- Misses        11750    12346     +596     
- Partials       5725     5989     +264     

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