server: block GracefulStop on method handlers and make blocking optional for Stop

[dfawley]

Fixes #6921

This reverts a behavior change in 1.60 for GracefulStop. Stop never blocked for the method handlers, so this would introduce a behavior change for Stop if we did it by default; instead we add a ServerOption to control the behavior.

RELEASE NOTES:

• server: fix GracefulStop to block until all method handlers return (v1.60 regression); add WaitForHandlers ServerOption (experimental) to cause Stop to also block until method handlers return (new feature).

[codecov]

Codecov Report

Merging #6922 (c61a4d7) into master (953d12a) will decrease coverage by 0.12%.
Report is 2 commits behind head on master.
The diff coverage is 100.00%.

❗ Current head c61a4d7 differs from pull request most recent head 8fc625b. Consider uploading reports for the commit 8fc625b to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6922      +/-   ##
==========================================
- Coverage   83.72%   83.60%   -0.12%     
==========================================
  Files         287      287              
  Lines       30835    30839       +4     
==========================================
- Hits        25816    25783      -33     
- Misses       3964     3989      +25     
- Partials     1055     1067      +12     

Files	Coverage Δ
server.go	81.43% <100.00%> (-0.03%)	⬇️

... and 20 files with indirect coverage changes

[sunjayBhatia]

LGTM, tested this out as well with my repro from issue #6921 and looks like GracefulStop works as expected and Stop works as expected when the new WaitForHandlers(true) option is passed to the server (but does not wait for handlers if that option is not passed).

[dfawley]

@sunjayBhatia great, thank you for confirming!