feat: cancel futures returned by .then()

[coryan]

This change is 

[codecov]

Codecov Report

Merging #166 into master will increase coverage by 0.06%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #166      +/-   ##
==========================================
+ Coverage   95.69%   95.76%   +0.06%     
==========================================
  Files          88       88              
  Lines        4021     4059      +38     
==========================================
+ Hits         3848     3887      +39     
+ Misses        173      172       -1

Impacted Files	Coverage Δ
google/cloud/internal/future_then_impl.h	100% <ø> (ø)	⬆️
google/cloud/future_generic_then_test.cc	99.19% <100%> (+0.02%)	⬆️
google/cloud/completion_queue_test.cc	94.23% <100%> (+0.64%)	⬆️
google/cloud/internal/future_impl.h	91.39% <100%> (+0.09%)	⬆️
google/cloud/completion_queue.cc	100% <100%> (ø)	⬆️
google/cloud/future_void_then_test.cc	99.58% <100%> (+0.01%)	⬆️
google/cloud/future_generic.h	100% <0%> (+3.03%)	⬆️

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[mr-salty]

Reviewed 6 of 6 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @tmatsuo)