gracefulswitch: Wait for all goroutines on close

[twz123]

Goroutines spawned during balancer swaps could outlive the call to Balancer.Close(). Monitor these via a wait group and wait for them to finish before returning from Close(). This prevents any noticeable side effects that could otherwise occur after Close() returns.

See:

• Allow waiting for all goroutines to exit on client connection close #8655
• clientconn: Wait for all goroutines on close #8666 (comment)

RELEASE NOTES:

• client: Closing a graceful switch balancer will now block until all pending goroutines complete.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.30%. Comparing base (647162c) to head (118c31b).
⚠️ Report is 9 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8746      +/-   ##
==========================================
- Coverage   83.38%   83.30%   -0.08%     
==========================================
  Files         418      418              
  Lines       32367    32371       +4     
==========================================
- Hits        26988    26968      -20     
+ Misses       4014     4011       -3     
- Partials     1365     1392      +27     

Files with missing lines	Coverage Δ
internal/balancer/gracefulswitch/gracefulswitch.go	72.41% <100.00%> (-14.06%)	⬇️

... and 29 files with indirect coverage changes

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