fix(pubsub): faster shutdowns for Publisher

[coryan]

Cancel the timer (if any) before shutting down. This can be a problem for applications that have very long batches (say multiple seconds or minutes).

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This change is 

[google-cloud-cpp-bot]

Google Cloud Build Logs
For commit: de1664f9ab00770f5e9437c1d79738e236e6e8ac

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[codecov]

Codecov Report

Base: 94.21% // Head: 94.22% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (de1664f) compared to base (fb40389).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #9991   +/-   ##
=======================================
  Coverage   94.21%   94.22%           
=======================================
  Files        1501     1501           
  Lines      140650   140674   +24     
=======================================
+ Hits       132520   132545   +25     
+ Misses       8130     8129    -1     

Impacted Files	Coverage Δ
...ud/pubsub/internal/batching_publisher_connection.h	100.00% <ø> (ø)
...d/pubsub/internal/batching_publisher_connection.cc	100.00% <100.00%> (ø)
...sub/internal/batching_publisher_connection_test.cc	99.44% <100.00%> (-0.18%)	⬇️
...e/cloud/spanner/testing/cleanup_stale_instances.cc	75.40% <0.00%> (-3.28%)	⬇️
...cloud/pubsub/internal/subscription_session_test.cc	98.49% <0.00%> (+0.50%)	⬆️
...integration_tests/schema_admin_integration_test.cc	100.00% <0.00%> (+1.11%)	⬆️

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[scotthart]

Reviewed 3 of 3 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @coryan)