ref(delayed rules): Don't use a decorator, call add_handler instead

[ceorourke]

It doesn't seem like any of the delayed processing is being called since it's not imported by anything, so this PR changes the registry pattern a bit so that it does.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.03%. Comparing base (8c9fbbd) to head (5d0f9b7).
Report is 2 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master   #70689       +/-   ##
===========================================
+ Coverage   50.46%   80.03%   +29.57%     
===========================================
  Files        6469     6506       +37     
  Lines      289328   290760     +1432     
  Branches    49888    50103      +215     
===========================================
+ Hits       146003   232713    +86710     
+ Misses     142892    57613    -85279     
- Partials      433      434        +1     

Files	Coverage Δ
src/sentry/buffer/redis.py	92.05% <100.00%> (+64.33%)	⬆️
src/sentry/rules/processing/delayed_processing.py	93.15% <100.00%> (+70.87%)	⬆️
src/sentry/tasks/post_process.py	92.23% <100.00%> (+78.48%)	⬆️

... and 2104 files with indirect coverage changes

[saponifi3d]

lgtm 🤞