Use ExecutorService for activating jobs

[remcowesterhoud]

Description

Activating jobs did not make use of the single threaded executor. This caused multiple threads to write at the same time, causing conflicts.

With this fix the synchronized is no longer necessary.

Related issues

closes #158

Definition of Done

Not all items need to be done depending on the issue and the pull request.

Code changes:

• The changes are backwards compatibility with previous versions
• If it fixes a bug then PRs are created to backport the fix

Testing:

• There are unit/integration tests that verify all acceptance criterias of the issue
• New tests are written to ensure backwards compatibility with further versions
• The behavior is tested manually

Documentation:

• Javadoc has been written
• The documentation is updated

[github-actions]

Unit Test Results

  87 files    87 suites   3m 11s ⏱️
186 tests 186 ✔️ 0 💤 0 ❌
558 runs  558 ✔️ 0 💤 0 ❌

Results for commit 063d9f5.

♻️ This comment has been updated with latest results.

[remcowesterhoud]

@Zelldon You were right about the activateJobs() method 👍. Would you mind reviewing this fix. I've also added a unit test which reproduces the issue.

[ChrisKujawa]

LGTM 👍