upcall: Delete flows that were not recently dumped.

Previously, we would clean up the ukeys whose flow was not seen in the
most recent dump, while leaving the flow in the datapath. In the
unlikely case that the datapath fails to dump a flow that still exists
in the datapath, this would cause double-counting of those flow stats.

This is currently very rare to see due to batching of datapath flow
deletion, but is more easily observable with upcoming patches which
modify the batch size based on dpif implementation.

Signed-off-by: Joe Stringer <joestringer@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>