Currently net::proxy::send() waits for previous packet to be sent to
another cpu before accepting next packet. This slows down sender to much.
This patch implement virtual queue that allows many packets to be send
simultaneously but it starts to drop packets when number of outstanding
packets goes over the limit, if we will try to queue them we will run
out of memory if a sender generates packets faster that they can be
sent. It also tries to generate back pressure by returning a feature
that will become ready when queue goes under the limit, but it returns
it only for a first sender that goes over it. Doing this for all callers
may be an option too, but it is not clear which one and how many should
we wake when queue goes under the limit again.
Only tx path is added for now. Will enable rx path when merge receive
buffer feature is supported in virtio-net.
UDP fragmentation offload is not hooked up, will do in a separate patch.
Only one cpu can talk to virtio NIC directly (at least without
multiqueue), so other cpus receive packets from the one that drives
virtio and forward packets that need to be send back. This is done
by proxy net interface.
