ike-sa-manager: Add a method to register/check out new IKE_SAs

ike-sa-manager: Add a method to register/check out new IKE_SAs

This way, jobs for new IKE_SAs (created via create_new()) may be
scheduled/queued before checkin() is called.  If they run before
that happens, they will now correctly block in checkout() instead of
doing nothing because the IKE_SA was not found.