Refactor scheduler to not use fcntl()

[Josephine-Rutten]

Is your feature request related to a problem? Please describe.
We use the package fcntl which is Linux based. This creates a problem for making the Dockers more lightweight and for people developing on a not-Linux machine. fcntl locks specific files. It's only used in the scheduler.py.

Describe the solution you'd like
We could replace the entire scheduler with the package apscheduler.

Other solutions retaining the scheduler
We will use a different packages. Online I found the following suggestions: portalocker, waitress. We can also check if the file locking necessary or if we would prefer to solve it in a different way.