rate_limiter: Migrate to diesel-async queries

[Turbo87]

one step closer to getting rid of these spawn_blocking() calls

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.17%. Comparing base (db62001) to head (5601672).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9992      +/-   ##
==========================================
+ Coverage   89.12%   89.17%   +0.05%     
==========================================
  Files         294      294              
  Lines       30593    30711     +118     
==========================================
+ Hits        27267    27388     +121     
+ Misses       3326     3323       -3     

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