lib/classifier: Use a prefix tree to optimize ports wildcarding.

Using a prefix tree (aka 'trie') for transport ports matching produces
less specific (more wildcarded) datapath megaflows.

Each subtable that matches on transport ports has it's own ports trie.
This trie is consulted only after a failing lookup to determine the
number of bits that need to be unwildcarded to guarantee that any
packet that should match on any of the other rules will not match this
megaflow.

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>