ofproto-dpif-xlate: Cache xlate_actions() effects.

This patch adds a new object called 'struct xlate_cache' which can be
set in 'struct xlate_in', and passed to xlate_actions() to cache the
modules affected by this flow translation. Subsequently, the caller can
pass the xcache to xlate_push_stats() to credit stats and perform side
effects for a lower cost than full flow translation.

These changes are aimed currently at long-lived flows, decreasing the
average dump duration for such flows by 50-80%. This allows more flows
to be supported in the datapath at a given time. Applying these changes
to short-lived flows is left for a later commit.

Signed-off-by: Joe Stringer <joestringer@nicira.com>
---
v1: Add caching for fin_timeout action.
    Expire netflows on xlate_cache_clear().
    Account to bonds using a copy of 'flow' rather than hash.
    Always build XC_NORMAL entry (previously only if may_learn is true)
    Rename xlate_from_cache()->xlate_push_stats()
    Add may_learn parameter to xlate_push_stats()
    Tidy up xlate_actions__() mirror/netflow code.
    Fold in style fixups.
RFC: First post.
revalidator: Only revalidate high-throughput flows.

Previously we would revalidate all flows if the "need_revalidate" flag
was raised. This patch modifies the logic to delete low throughput flows
rather than revalidate them. High-throughput flows are unaffected by
this change. This patch identifies the flows based on the mean time
between packets since the last dump.

This change is primarily targeted at situations where:
* Flow dump duration is high (~1 second)
* Revalidation is triggered. (eg, by bridge reconfiguration or learning)

After the need_revalidate flag is set, next time a new flow dump session
starts, revalidators will begin revalidating the flows. This full
revalidation is more expensive, which significantly increases the flow
dump duration. At the end of this dump session, the datapath flow
management algorithms kick in for the next dump:

* If flow dump duration becomes too long, the flow limit is decreased.
* The number of flows in the datapath then exceeds the flow_limit.
* As the flow_limit is exceeded, max_idle is temporarily set to 100ms.
* Revalidators delete all flows that haven't seen traffic recently.

The effect of this is that many low-throughput flows are deleted after
revalidation, even if they are valid. The revalidation is unnecessary
for flows that would be deleted anyway, so this patch skips the
revalidation step for those flows.

Note that this patch will only perform this optimization if the flow has
already been dumped at least once, and only if the time since the last
dump is sufficiently long. This gives the flow a chance to become
high-throughput.

Signed-off-by: Joe Stringer <joestringer@nicira.com>
---
v1: Determine "high-throughput" by packets rather than bytes.
    Calculate the mean time between packets for comparison, rather than
      comparing the number of packets since the last dump.
RFC: First post.