ofproto-dpif: Test flow stats reporting.

Basic test to check that the datapath reports the correct number of
packets seen, after a delay of 1 second.

Signed-off-by: Joe Stringer <joestringer@nicira.com>
upcall: Defer ukey deletion until after pushing stats.

It is possible for a datapath to dump the same flow twice, for instance
if the flow is the last in a batch of flows to be dumped, then a new
flow is inserted into the same bucket before the flow dumper fetches
another batch.

In this case, datapath flow stats may be duplicated: The revalidator
records the stats from the first flow, using the ukey to get the stats
delta. The ukey is deleted, then the revalidator reads the second
(duplicate) flow and cannot lookup the ukey for the delta. As such, it
will push the stats as-is.

This patch reduces the likelihood of such stats duplications by
deferring ukey deletion until after stats are pushed for deleted flows.

Signed-off-by: Joe Stringer <joestringer@nicira.com>
upcall: Refactor ukey creation and dump handling

This splits out functions for re-use by later patches, and compacts the
udump revalidation code.

Co-authored-by: Ethan Jackson <ethan@nicira.com>
Signed-off-by: Joe Stringer <joestringer@nicira.com>
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>
upcall: Remove datapath flows when setting n-threads.

Previously, we would delete all ukeys when changing the number of
threads, but leave all flows in the datapath. This would cause
double-counting of stats for any flows that remain in the datapath. This
patch fixes the issue by ensuring that all flows are deleted from the
datapath before changing the number of threads.

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