Draft: Reread blf file instead of storing all messages in a buffer

Description

For smaller files the concept of storing the relevant messages in a buffer and iterating again over them is fine. For larger files this leads to memory problems. So read the file twice. This is probably slower but less memory is required

Conformity

• Changelog entry
• Unit tests Already covered
• Update INSTALL
• Downport
• Fixes:

When external dependencies are removed

• Reporting to ...