Do not leak _unique_id out of amqp drivers

In commit d8d2ad9 we delayed when a message's unique ID gets added to
the duplicate message cache in order to allow for message requeueing.

However, as part of this, we exposed the private _unique_id field
outside of the driver. This commit reverses that change by storing
the ID in the AMQPIncomingMessage object.

Change-Id: Ibeb7896de7ad9abf3c6a43495c1a87aabb762c0d
Merge "Do not leak _unique_id out of amqp drivers"