ike-rekey: Reset IKE_SA on the bus after destroying new IKE_SA

ike-rekey: Reset IKE_SA on the bus after destroying new IKE_SA

The destroy() method sets the IKE_SA on the bus to NULL, we reset it to
the current IKE_SA so any events and log messages that follow happen in
the correct context.

A practical example where this is problematic is a DH group mismatch,
which causes the first CREATE_CHILD_SA exchange to fail.  Because the SA
was not reset previously, the message() hook for the CREATE_CHILD_SA
response, for instance, was triggered outside the context of an IKE_SA,
that is, the ike_sa parameter was NULL, which is definitely not expected
by several plugins.

Fixes #862.