child-sa: Replace reqid based marks by "unique" marks

child-sa: Replace reqid based marks by "unique" marks

As we now use the same reqid for multiple CHILD_SAs with the same selectors,
having marks based on the reqid makes not that much sense anymore. Instead we
use unique marks that use a custom identifier. This identifier is reused during
rekeying, keeping the marks constant for any rule relying on it (for example
installed by updown).

This also simplifies handling of reqid allocation, as we do not have to query
the marks that is not yet assigned for an unknown reqid.