cache_req: move from switch to plugins; add logic cache_req grown quite big from the original code and it turned out that using switch statements to branch code for different cases makes the code quite hard to read and further extend and any modification to the logic itself is difficult. This patch changes the switch statements to plugins with small functions and separates logic into multiple modules. This gives us better control over the code and improves readability and maintainability while keeping code duplication to minimum. Reviewed-by: Jakub Hrozek <jhrozek@redhat.com> Reviewed-by: Petr Cech <pcech@redhat.com>