ikev2: handoff packets

ikev2: handoff packets

current approach saves state in per-thread data structure. in
multi-worker + nat-t cases udp/500 and udp/4500 might be dispatched on
different workers. this patch adds hands off packet to 1 explicit thread
- 1st worker (or main thread in case there're no workers) or to thread
  that was explicitly set by user via configuration

Type: improvement

Change-Id: Ib5cd9a4b8612dfaa63b276035709524f7a492d4f
Signed-off-by: Stanislav Zaikin <[email protected]>