core: correlation-id not generated when loggingFlushDisabled == true

[SuperITMan]

I'm submitting a...

[ ] Regression (a behavior that used to work and stopped working in a new release)
[X] Bug report  
[ ] Feature request
[ ] Documentation issue or request
[ ] Support request => Please do not submit support request here, instead see https://github.com/NationalBankBelgium/stark/blob/master/CONTRIBUTING.md#got-a-question-or-problem

Current behavior

Disabling the loggingFlush breaks the keep-alive since the correlation-id is not generated.

Expected behavior

The KeepAlive service should be usable without enabling the loggingFlush.

Minimal reproduction of the problem with instructions

In the file "stark-app-config.json", set the property loggingFlushDisabled: true with keepAlive enabled.
Start the application and check the request headers for the keep-alive requests. The header "correlation-id" is undefined.

What is the motivation / use case for changing the behavior?

Solve a bug.

Environment

Angular version: 7+
Stark version: 10+


Others: