clientconn: Wait for all goroutines on close

[twz123]

Three goroutines could outlive a call to ClientConn.close(). Add mechanics to cancel them and wait for them to complete when closing a client connection.

Fixes #8655.

RELEASE NOTES:

• Closing a client connection will cancel all pending goroutines and block until they complete.

[codecov]

Codecov Report

❌ Patch coverage is 91.35802% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.99%. Comparing base (58d4b2b) to head (e45e9d7).

Files with missing lines	Patch %	Lines
clientconn.go	84.09%	5 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8666      +/-   ##
==========================================
+ Coverage   81.76%   81.99%   +0.22%     
==========================================
  Files         417      417              
  Lines       40806    40858      +52     
==========================================
+ Hits        33367    33501     +134     
+ Misses       6057     5973      -84     
- Partials     1382     1384       +2     

Files with missing lines	Coverage Δ
balancer_wrapper.go	86.56% <100.00%> (+2.88%)	⬆️
internal/balancer/gracefulswitch/gracefulswitch.go	86.22% <100.00%> (-1.17%)	⬇️
internal/testutils/pipe_listener.go	84.44% <100.00%> (+2.39%)	⬆️
clientconn.go	90.40% <84.09%> (+1.86%)	⬆️

... and 18 files with indirect coverage changes

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