[Prime] Detect pubsub duplicate messages from OCS gateway

[mpeterss]

Cloud Pub/Sub guarantees at-least-once message delivery, which means that occasional duplicates are to be expected. In the OCS gateway we detect duplicates as the request-Id is tracked and if we get a duplicate then it is ignored. But on the Prime side there is no detection, this means that there is a possibility for double charging in the case where a message from Ocs gateway to Prime is duplicated.