[Dataflow Streaming] Enabled Heartbeat by Default

[TongruiLi]

This sets heartbeat work by default and will rollout along next beam release. This CL also refactors and fixed some unit test issues.

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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 71.37%. Comparing base (5a09095) to head (5c1fa0e).

:exclamation: Current head 5c1fa0e differs from pull request most recent head 3f97733

Please upload reports for the commit 3f97733 to get more accurate results.

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #31689      +/-   ##
============================================
+ Coverage     71.18%   71.37%   +0.19%     
+ Complexity     3019     1474    -1545     
============================================
  Files          1058      900     -158     
  Lines        134254   114202   -20052     
  Branches       3257     1076    -2181     
============================================
- Hits          95571    81515   -14056     
+ Misses        35536    30659    -4877     
+ Partials       3147     2028    -1119     

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[TongruiLi]

assign set of reviewers

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Reviewers are already assigned to this PR: @robertwb