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>