lib/dpif-netdev: Integrate megaflow classifier.

Megaflow inserts and removals are simplified:

- No need for classifier internal mutex, as dpif-netdev already has a
  'flow_mutex'.
- Number of memory allocations/frees can be halved.
- Lookup code path can rely on netdev_flow_key always having inline data.

This will also be easier to simplify further when moving to per-thread
megaflow classifiers in the future.

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Acked-by: Alex Wang <alexw@nicira.com>