[AIRFLOW-1771] Rename heartbeat to avoid confusion

[saguziel]

Dear Airflow maintainers,

Please accept this PR. I understand that it will not be reviewed until I have checked off all the steps below!
@aoen @bolkedebruin

JIRA

• My PR addresses the following Airflow JIRA issues and references them in the PR title. For example, "[AIRFLOW-XXX] My Airflow PR"
  • https://issues.apache.org/jira/browse/AIRFLOW-1771

Description

• Here are some details about my PR, including screenshots of any UI changes: People confuse "Boom" with something going bad (Boom sounds like an explosion, not a heartbeat). We should rename this to avoid confusion

Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason: n/a

Commits

• My commits all reference JIRA issues in their subject lines, and I have squashed multiple commits if they address the same issue. In addition, my commits follow the guidelines from "How to write a good git commit message":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

will add the jira issue in a sec

[aoen]

LGTM

[codecov-io]

Codecov Report

Merging #2743 into master will increase coverage by 0.07%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2743      +/-   ##
==========================================
+ Coverage      73%   73.07%   +0.07%     
==========================================
  Files         155      155              
  Lines       11835    11835              
==========================================
+ Hits         8640     8649       +9     
+ Misses       3195     3186       -9

Impacted Files	Coverage Δ
airflow/jobs.py	79.87% <100%> (+0.26%)	⬆️
airflow/utils/helpers.py	56.32% <0%> (+2.87%)	⬆️
airflow/task_runner/bash_task_runner.py	100% <0%> (+6.66%)	⬆️

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