[17.12] [manager/dispatcher] Synchronize Dispatcher.Stop() with incoming rpcs.

[anshulpundir]

git cherry-pick 6b163ff
Cherry pick was clean.

[andrewhsu]

@anshulpundir needs -x flag for cherry-pick:

$ git cherry-pick -s -x 6b163ff

[codecov]

Codecov Report

Merging #2522 into bump_v17.12 will decrease coverage by 5.5%.
The diff coverage is 85.18%.

@@               Coverage Diff               @@
##           bump_v17.12    #2522      +/-   ##
===============================================
- Coverage        67.27%   61.76%   -5.51%     
===============================================
  Files               80      129      +49     
  Lines            11488    21247    +9759     
===============================================
+ Hits              7728    13124    +5396     
- Misses            2969     6709    +3740     
- Partials           791     1414     +623

[andrewhsu]

LGTM when jobs are green

[dperny]

LGTM.

[andrewhsu]

For reference, this is a backport of PR:

• [manager/dispatcher] Synchronize Dispatcher.Stop() with incoming rpcs. #2519 [manager/dispatcher] Synchronize Dispatcher.Stop() with incoming rpcs.