Update async_dofn.py

[dustin12]

Fixes two problems found in async during load testing:

1. We should not do any blocking operations under a lock in commit_finished_items. Add items outside the lock.
2. Timers should be skewed so they don't all fire at once.

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[gemini-code-assist]

Summary of Changes

Hello @dustin12, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses two critical performance issues identified during load testing of asynchronous operations. It refactors the AsyncWrapper to prevent blocking operations from occurring under a lock, thereby improving concurrency and responsiveness. Additionally, it introduces a mechanism to skew timer firing times, mitigating the risk of multiple timers activating simultaneously and causing performance spikes. These changes collectively enhance the stability and efficiency of asynchronous processing within the system.

Highlights

• Concurrency Control Improvement: Blocking operations (rescheduling items) within a critical section (commit_finished_items under a lock) have been moved outside the lock to prevent performance bottlenecks during high load.
• Timer Collision Prevention: Timers are now scheduled with a random offset to distribute their firing times, avoiding simultaneous activations that could lead to resource contention and performance degradation.
• Reduced Busy-Waiting: The sleep duration in schedule_item when waiting for buffer availability has been significantly reduced from 1 second to 0.01 seconds, making the waiting process more responsive.

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[codecov]

Codecov Report

❌ Patch coverage is 10.00000% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 40.21%. Comparing base (db92a3a) to head (2364455).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
sdks/python/apache_beam/transforms/async_dofn.py	10.00%	9 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             master   #36586       +/-   ##
=============================================
- Coverage     56.93%   40.21%   -16.72%     
  Complexity     3394     3394               
=============================================
  Files          1222     1222               
  Lines        186787   186911      +124     
  Branches       3545     3545               
=============================================
- Hits         106348    75172    -31176     
- Misses        77067   108367    +31300     
  Partials       3372     3372               

Flag	Coverage Δ
python	40.51% <10.00%> (-40.50%)	⬇️

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