nat: fix EI hairpinning thread safety

nat: fix EI hairpinning thread safety

Avoid doing inter-thread reads without locks by doing a handoff before
destination address rewrite. Destination address is read from a session
which is possibly owned by a different thread. By splitting the work in
two parts with a handoff in the middle, we can do both in a thread safe
way.

Type: improvement
Signed-off-by: Klement Sekera <ksekera@cisco.com>
Change-Id: I1c50d188393a610f5564fa230c75771a8065f273