revalidator: Improve optimization to skip revalidation.

The should_revalidate() optimisation introduced with commit 698ffe3623
(revalidator: Only revalidate high-throughput flows.) was a little
aggressive, occasionally deleting flows even when OVS is quite capable
of performing full revalidation.

This commit modifies the logic to:
* Firstly, check if we are likely to handle full revalidation, and
  attempt that instead.
* Secondly, fall back to the existing flow throughput estimations to
  determine whether to revalidate the flow or just delete it.

VMware-BZ: #1271926

Signed-off-by: Joe Stringer <joestringer@nicira.com>
Acked-by: Alex Wang <alexw@nicira.com>