classifier: Support miniflow as a key.

Support struct miniflow as a key for datapath flow lookup.

The new classifier interface classifier_lookup_miniflow_first() takes
a miniflow as a key and stops at the first match with no regard to
flow prioritites.  This works only if the classifier has no
conflicting rules (as is the case with the userspace datapath
classifier).

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Reviewed-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>