revalidator: Defer stats push to end of validation.

revalidator: Defer stats push to end of validation.

To make more of the core revalidate() functions do just one thing and
not modify state on the way, refactor them to prepare the xcache then
defer the ukey modification and stats/side effects execution to the end
of successful revalidation.

If revalidation causes deletion, then the xcache will be prepared and
attached to the ukey, but the actual execution will be skipped since it
will be executed on flow_delete very soon anyway with final stats.

Signed-off-by: Joe Stringer <joe@ovn.org>
Acked-by: Daniele Di Proietto <diproiettod@vmware.com>