dpif-netdev: Implement batched flow dumping.

Previously, flows were retrieved one by one when dumping flows for
datapaths of type 'netdev'. This increased contention for the dump's
mutex, negatively affecting revalidator performance.

This patch retrieves batches of flows when dumping flows for datapaths
of type 'netdev'.

Signed-off-by: Ryan Wilson <wryan@nicira.com>
[blp@nicira.com relaxed max_flows restriction]
Signed-off-by: Ben Pfaff <blp@nicira.com>