Reactive cache flow goes through the background worker which causes
arguments to the PacketFlowQuery to be stringified. This causes
reactive cache worker to use a different cache key from the one used
by the web process, as a result background worker will not re-calc
cache. This patch converts PacketFlowQuery to use arguments that will
work consistently for both flows. This patch also updates controller
to send :accepted on initial caching request to improve frontend
polling logic.