Refactoring smells

[tapanxd]

Brief

There are a total of 3 changes done in this PR

• Refactored complex conditional statement in oneExecutionDone method inside:
  com.github.kagkarlsson.scheduler > DueExecutionsBatch > oneExecutionDone

• Long method named updateHeartbeats() has been refactored using extract method technique.
  com.github.kagkarlsson.scheduler > Scheduler > updateHeartbeats

• Renamed the variable name so that it better reflects the method's intent or variable's purpose clearly.
  com.github.kagkarlsson.scheduler > Scheduler > delegate

Fixes

It does not necessarily fix any issues, this pull requests solves a few implementation smells in the code.

Reminders

• Added/ran automated tests
• Update README and/or examples
• Ran mvn spotless:apply

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cc @kagkarlsson

[tapanxd]

Hi @kagkarlsson, can you provide a feedback on this PR?

[kagkarlsson]

Appreciate the effort. Not exactly sure when I will have the bandwidth to have a look at it