fix(pubsub): avoid deadlocks in publish flow control

[coryan]

Fixes #7309

---

This change is 

[google-cloud-cpp-bot]

Google Cloud Build Logs
For commit: bef904ecec05fdd867621b0821d7386ef704abb0

• Public Access Build Logs
• Cloud Build Console (requires auth)
• Kokoro Logs (requires auth)

ℹ️ NOTE: Kokoro logs are linked from "Details" below.

[codecov]

Codecov Report

Merging #7313 (bef904e) into main (25a331f) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #7313   +/-   ##
=======================================
  Coverage   93.69%   93.69%           
=======================================
  Files        1347     1347           
  Lines      116544   116562   +18     
=======================================
+ Hits       109194   109212   +18     
  Misses       7350     7350           

Impacted Files	Coverage Δ
...b/internal/flow_controlled_publisher_connection.cc	100.00% <100.00%> (ø)
...ernal/flow_controlled_publisher_connection_test.cc	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 25a331f...bef904e. Read the comment docs.

[dbolduc]

Reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @coryan)