Improve sleeve performance in mixed sleeve/fast datapath deployments

[awh]

As seen here, sleeve performance is penalised in mixed deployments due to the use of the ODP miss handler instead of mmap'ed libpcap. Possible fixes:

• Contribute mmap'ed netlink support to OVS
• Use pcap and fast datapath simultaneously, inserting a flow rule that directs future traffic to a pcap'ed interface where we would currently veto flow creation due to the presence of sleeve flow ops