Avoid creating handler waiter until shutdown

[bdraco]

What do these changes do?

Every request created _handler_waiter. Instead only create it when we need to wait for graceful shutdown. Avoids one future creation for every request if we are not shutting down.

Are there changes in behavior for the user?

no

Is it a substantial burden for the maintainers to support this?

no

related issue #2779

profile

before

after

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.31%. Comparing base (98b363e) to head (94fcfef).
Report is 1 commits behind head on master.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #9168   +/-   ##
=======================================
  Coverage   98.31%   98.31%           
=======================================
  Files         107      107           
  Lines       34485    34489    +4     
  Branches     4095     4096    +1     
=======================================
+ Hits        33903    33907    +4     
  Misses        411      411           
  Partials      171      171           

Flag	Coverage Δ
CI-GHA	98.20% <100.00%> (+<0.01%)	⬆️
OS-Linux	97.87% <100.00%> (+<0.01%)	⬆️
OS-Windows	96.28% <100.00%> (+<0.01%)	⬆️
OS-macOS	97.54% <100.00%> (+<0.01%)	⬆️
Py-3.10.11	97.64% <100.00%> (+<0.01%)	⬆️
Py-3.10.14	97.58% <100.00%> (+<0.01%)	⬆️
Py-3.11.10	97.47% <100.00%> (?)
Py-3.11.9	97.53% <100.00%> (-0.28%)	⬇️
Py-3.12.5	97.58% <100.00%> (+<0.01%)	⬆️
Py-3.12.6	97.64% <100.00%> (+<0.01%)	⬆️
Py-3.9.13	97.53% <100.00%> (+<0.01%)	⬆️
Py-3.9.19	97.47% <100.00%> (+<0.01%)	⬆️
Py-pypy7.3.16	97.08% <100.00%> (+<0.01%)	⬆️
VM-macos	97.54% <100.00%> (+<0.01%)	⬆️
VM-ubuntu	97.87% <100.00%> (+<0.01%)	⬆️
VM-windows	96.28% <100.00%> (+<0.01%)	⬆️

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