tcp_proxy now uses an AF_PACKET socket as the FD for netstack link layer
endpoint instead of a tap device. It also changes the link layer endpoint to use
PacketMMap dispatch instead of Readv. This reduces overall cpu and reflects the
current runsc setup which uses PacketMMap and also uses veth devices to receive
packets.
Also fixed a bug in gonet where Read() was not doing coalescing read and would
read small amounts at a time.
PiperOrigin-RevId: 234714768
Change-Id: Idabf8e600e4512489d3ba441c4096dc74deba5d7
3e3a1ef9d6