Fix serialization of update notification payload

Fix serialization of update notification payload

Fix Gson serialization of ChampEventEnvelope:
- serialize key as primitive.
  Currently output as object ({}) with a "value" property.
  This is due to Optional object declaration and misalignment between
  Gson serializer and Jackson annotations in ChampObject
- serialize transactionId as transaction-id
- serialize dbTransactionId as db-transaction-id

Change-Id: I3a9824ad376ca8189ba31cc9442cb42bb06a9d70
Issue-ID: AAI-1218
Signed-off-by: Michael Arrastia <MArrasti@amdocs.com>