Classifier: Staged subtable matching.

Subtable lookup is performed in ranges defined for struct flow,
starting from metadata (registers, in_port, etc.), then L2 header, L3,
and finally L4 ports.  Whenever it is found that there are no matches
in the current subtable, the rest of the subtable can be skipped.  The
rationale of this logic is that as many fields as possible can remain
wildcarded.

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>