If a Quick mode is initiated for a CHILD_SA that is already installed
we can identify this situation and rekey the already installed CHILD_SA.
Otherwise we end up with several CHILD_SAs in state INSTALLED which
means multiple calls of child_updown are done. Unfortunately,
the deduplication code later does not call child_updown() (so up and down
were not even).
Closesstrongswan/strongswan#95.
