classifier: Simplify versioning.

classifier: Simplify versioning.

After all, there are some cases in which both the insertion version
and removal version of a rule need to be considered.  This makes the
cls_match a bit bigger, but makes classifier versioning much simpler
to understand.

Also, avoid using type larger than int in an enum, as it is not
portable C.

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>