Make sure grape-engine exit when CTRL-C without crash

[sighingnow]

What do these changes do?

• Reset the pointer to prevent double-shutdown which causes crash inside grpc
• Terminate dispatcher (and its queues) correctly when closing

Related issue number

Fixes #2749

[codecov-commenter]

Codecov Report

Merging #2761 (852bed9) into main (df16284) will increase coverage by 32.22%.
The diff coverage is n/a.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2761       +/-   ##
===========================================
+ Coverage   41.39%   73.61%   +32.22%     
===========================================
  Files          99       99               
  Lines       10381    10392       +11     
===========================================
+ Hits         4297     7650     +3353     
+ Misses       6084     2742     -3342     

see 57 files with indirect coverage changes

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