Similar to the FETCHMAIL_PARALLEL option which enables the use of IMAP IDLE, this PR provides the same functionality for getmail.
It builds upon the existing code as it doesn't touch the config file generation. Instead it only focusses on the start of getmail.
This should provide the functionality requested in discussion #4667

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• [ x ] Improvement (non-breaking change that does improve existing functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ x ] This change requires a documentation update

Checklist

• [ x ] My code follows the style guidelines of this project
• [ x ] I have performed a self-review of my code
• [ x ] I have commented my code, particularly in hard-to-understand areas
• [ x ] I have made corresponding changes to the documentation (README.md or the documentation under docs/)
• [ x ] If necessary, I have added tests that prove my fix is effective or that my feature works
• [ x ] New and existing unit tests pass locally with my changes
• [ x ] I have added information about changes made in this PR to CHANGELOG.md